Add Git commit message article

.github/CONTRIBUTING.md

@@ -336,6 +336,9 @@ other bugs and features to work on.
 If the patch fixes a bug introduced after the last release, it should not get
 a change log entry.
 
+If you haven't already, read the excellent article [How to Write a Git Commit
+Message](https://chris.beams.io/posts/git-commit/).
+
 
 ## Contributing a feature
 
@@ -373,6 +376,9 @@ If it adds a new OpenGL, OpenGL ES or Vulkan option or extension, support
 for it must be added to `tests/glfwinfo.c` and the behavior of the library when
 the extension is missing documented in `docs/compat.dox`.
 
+If you haven't already, read the excellent article [How to Write a Git Commit
+Message](https://chris.beams.io/posts/git-commit/).
+
 Features will not be rejected because they don't include all the above parts,
 but please keep in mind that maintainer time is finite and that there are many
 other features and bugs to work on.