Add section for building upstream branches in README.md

Author: stlehmann

README.md

@@ -38,3 +38,29 @@ The following modules are supported by PyQt5-stubs:
 * QtTest
 * QtXml
 * sip
+
+# Building upstream stubs
+The Dockerfile is used to build all of the stubs for the upstream PyQt5 modules.
+The Dockerfile consists of multiple build layers:
+* core: `PyQt5`
+* `PyQt3D`
+* `PyQtChart`
+* `PyQtDataVisualization`
+* `PyQtPurchasing`
+* `PyQtWebEngine`
+* an output layer
+
+Each module build layer deposits its stub files within `/output/` in its
+filesystem. The output layer then collects the contents of each into its own 
+`/output/` dir for export to the host computer. Build args are provided to 
+change the version of each module.
+
+A convenience script, `build_upstream.py`, is provided. It builds the stubs and 
+copies them to the host computer. Make sure you install `docker-py` to use it.
+It builds `$PWD/Dockerfile` (overridden with `--dockerfile`) and outputs the
+stubs to `$PWD/PyQt5-stubs` (overridden with `--output-dir`). 
+
+\* There are a few missing modules: `QtAxContainer`, `QtAndroidExtras`, 
+`QtMacExtras`, and `QtWindowsExtras`. The current project understanding is that
+they need to be built on the target platform, something a Linux-based docker
+image cannot do. The deprecated `Enginio` module is also missing.