docs: clarify CC trailer placement and PR description usage

New GitGitGadget users sometimes put CC: trailers in their commit
messages (where they are ignored) instead of in the Pull Request
description (where GitGitGadget reads them).  They also sometimes
duplicate the commit message in the PR description, not realizing
that the PR description appears below the three-dash line in the
emailed patch.

Add two bullet points to the usage instructions to help avoid
these common mistakes.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Tarjan <github@paulisageek.com>

Author: ptarjan

content/_index.md

@@ -25,6 +25,9 @@ So you cloned [https://github.com/git/git](https://github.com/git/git) and imple
   CC: Revi Ewer <revi.ewer@some.domain>, Ill Takalook <ill.takalook@other.domain>
   ```
 
+- **Important:** Do not write `CC:` trailers in your commit message. GitGitGadget does not read them there. Put `CC:` lines in the Pull Request description instead.
+- The Pull Request description appears below the `---` line in the emailed patch. This area is for supplementary information (e.g. a changelog between iterations) that is not part of the permanent commit history. Avoid duplicating your commit message here.
+
 You will also want to read [Git's contribution guidelines](https://git-scm.com/docs/SubmittingPatches) to make sure that your contributions are in the expected form, as well as the project's [coding guidelines](https://github.com/git/git/blob/master/Documentation/CodingGuidelines). You might also want to read the [gitworkflows](https://git-scm.com/docs/gitworkflows) manual page to understand how your contributions will be integrated in Git's repository, as well as [this note from Git's maintainer](https://github.com/git/git/blob/todo/MaintNotes).
 
 The first time you use GitGitGadget, you need to be added to the list of users with permission to use GitGitGadget (this is a <i>very</i> simple anti-spam measure). Using <a src="https://web.libera.chat/#git-devel">IRC</a> or <a src="https://github.com/gitgitgadget/git/pulls?utf8=%E2%9C%93&q=is%3Apr+is%3Aopen+%22%2Fallow%22">PRs</a> you can find a user already on this list and request them to add you to the list. To do so, they simply have to add a comment to that Pull Request that says <code>/allow</name></code>.