Add contributing guidelines

CONTRIBUTING.md

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+# Contributing Guidelines
+Contributions are welcome! This document provides some resources and guidelines to help with the process.
+
+Codex collects related Unicode symbols as different *variants* of the same *symbol*.[^symbol]
+For example, `→ ⇒ ↑ ⇑` are four variants of the `arrow` symbol.
+Each symbol has a default variant (here `→`).
+To refer to a particular variant, *modifiers* can be appended to the symbol name
+using dot separators.
+For example `⇒` is `arrow.r.double`, `↑` is `arrow.t` and `⇑` is `arrow.t.double`.
+Modifiers are order-independent, so the latter can also be referred to as `arrow.double.t`.
+Additionally, not all modifiers have to be specified, in which case the best match[^match]
+will be taken. For example, `⇒` can also be referred to as `arrow.double`.
+Groups of related symbols are collected into *modules*. Modules can also contain other modules.
+Codex exports two top-level modules: `sym` for text-style symbols and `emoji` for emoji;
+Their source code is found in `src/modules/`.
+
+If you need help with a contribution, you can also ask us [on Discord](https://discord.com/channels/1054443721975922748/1277628305142452306).
+
+Proposals used to be written in a dedicated Proposals document,
+but new proposals should now be filed as GitHub issues instead.
+The [document](https://typst.app/project/riXtMSim5zLCo7DWngIFbT)
+has been repurposed to serve as a collection of useful information
+and naming ideas.
+
+[^symbol]: This document also uses "symbol" in the more abstract sense of a graphical symbol.
+[^match]: See the documentation of `ModifierSet::best_match_in` for the exact details.
+
+## Pull Requests
+- All PRs require two approvals by collaborators to be merged.
+- PRs with breaking changes require three such approvals instead.
+- PRs with changes to the public Rust API also require an approval by @laurmaedje.
+
+To remove a symbol or variant, it is first marked as deprecated
+(This is considered a breaking change).
+After a Typst version that includes this deprecation notice has been released,
+the deprecated symbol or variant will be removed (This is not considered a breaking change).
+Instead of being removed, the name can also be repurposed for a different symbol,
+which can be seen as a combination of removing the old symbol or variant
+and adding a new one with the same name.
+
+## Conventions
+When adding new modules, symbols or variants, please try to be consistent with
+existing ones. Below are some guidelines based on existing symbols. These aren't
+always hard rules, especially because of how messy Unicode can be, but you should
+adhere to them if possible.
+
+### General Conventions
+- English words use US spelling.
+- Modifier and module names are entirely lowercase.
+- Symbol names are lowercase unless the symbol is an uppercase letter.
+- Symbol names should be at least two characters long so they can be used easily in Typst's math mode.
+- When a symbol is added to a base, the symbol name is used as a modifier on the base.[^modifname]
+    This can have the following meanings:
+    1. The symbol is added around or inside the base as a subordinate (smaller than the base),
+        e.g. `eq.quest`, `triangle.stroked.dot`.
+    2. The symbol is stacked below the base, e.g. `gt.lt`.
+    3. The symbol is stacked to the right of the base, e.g. `colon.eq`.
+    4. The symbol is overlaid at the center of the base, e.g. `integral.dash`.
+    5. The symbol surrounds the base, e.g. `plus.square`.
+- Notable exceptions to the previous convention:
+    - When `.eq` is used in the second sense (stacked below), it only adds a single line and not two,
+        e.g. `lt.eq`. For two lines below, `.equiv` is used, e.g. `lt.equiv`.
+
+[^modifname]: Though a modifier can also just coincidentally be a symbol name, e.g. `.not`.
+
+### Established Generic Modifiers
+These have a broad meaning and can have varying interpretations.
+<!-- Geometry -->
+- `.l`/`.r`/`.t`/`.b`: The four main directions (left/right/top/bottom), e.g. `arrow.l`, `times.r`.
+    - For delimiters, `.l` means opening and `.r` means closing, e.g. `paren.l`, `quote.r`.
+- `.tl`/`.tr`/`.bl`/`.br`: The four corners, e.g. `arrow.tl`, `triangle.stroked.tr`.
+    - Generally, these are used for a single, diagonal direction,
+        whereas combinations of two main directions (like `.t.l`) are used to mean both of them at once,
+        e.g. `arrow.t.l`, if it existed, would be a bidirectional arrow that points both top and left,
+        similarly to how `arrow.l.r` is an arrow pointing both left and right.
+- `.cw`/`.ccw`: Clockwise/Counterclockwise, e.g. `arrow.cw`, `integral.ccw`.
+- `.tiny`/`.small`/`.medium`/`.big`: A geometric shape with a certain size, e.g. `square.stroked.small`.
+<!-- Strokes -->
+- `.stroked`/`.filled`: A symbol that has an empty/filled interior, e.g. `circle.stroked`, `arrow.r.filled`.
+    (They correspond to Unicode's "white"/"black".)
+- `.dotted`: A shape with a dotted line instead of a full stroke, e.g. `circle.dotted`.
+- `.light`/`.heavy`: A shape with a certain stroke weight, e.g. `checkmark.heavy`.
+<!-- Other (in alphabetic order) -->
+- `.alt`: An alternate glyph for the symbol, e.g. `phi.alt`.
+- `.double`, `.triple`, `.quad`: A symbol that has 2-4 of something, e.g. `excl.double`, `eq.quad`.
+
+### Established Concrete Modifiers
+These have a specific meaning that is not open to much interpretation.
+<!-- (in alphabetic order) -->
+- `.big`: A [large](https://www.unicode.org/Public/math/latest/MathClassEx-15.html) (n-ary) version
+    of an operator, e.g. `union.big`.
+- `.inv`: Either vertically mirrored or a 180° rotated version of a symbol, e.g. `amp.inv`, `Omega.inv`.
+    - See also [#108](https://github.com/typst/codex/issues/108).
+- `.not`: A negation of the symbol, e.g. `eq.not`.
+- `.o`: A symbol with a circle around it, e.g. `plus.circle`.
+    - See also [#62](https://github.com/typst/codex/pull/62)
+- `.rev`: A horizontally mirrored version of a symbol, e.g. `in.rev`.
+    - See also [#108](https://github.com/typst/codex/issues/108).
+- `.sq`: A "squarified" version of a symbol, e.g. `subset.sq`.
+    - See also [#110](https://github.com/typst/codex/pull/110)