message: Add a note on commit messages to our developer guide
diff: @@ -20,6 +20,10 @@ LLVM style guide):
  recursive way). At the moment, we tolerate it for the two following cases:
  * The nesting of the IR: we use recursion when traversing nested regions.
  * Type nesting: recursion may be used for the nesting of composite types.
  +* Follow the `git` conventions for writing a commit message, in particular the
  + first line is the "title", it should be followed by an empty line and an
  + optional description. This [post](https://chris.beams.io/posts/git-commit/)
  + give examples and more details.
  Please run clang-format on the files you modified with the `.clang-format`
  configuration file available in the root directory. Check the clang-format
  