Add a CONTRIBUTING.md file

Collect some conventions about contributing to Yasnippet (which have
been scattered amongst various issues/pull request) into a single file.

* CONTRIBUTING.md: New File.

CONTRIBUTING.md

@@ -0,0 +1,27 @@
+# Submitting Bug Reports
+
+Please read [Important note regarding bug reporting][bugnote].
+
+# Contributing to Yasnippet
+
+## Copyright Assignment
+
+Yasnippet is part of GNU ELPA, so it falls under the same copyright
+assignment policy as the rest of Emacs (see "Copyright Assignment" in
+https://www.gnu.org/software/emacs/CONTRIBUTE). A copyright assignment
+for Emacs also covers Yasnippet.
+
+## Commit message format
+
+The commit message format roughly follows Emacs conventions, although
+there is no separate Changelog file.
+
+    The commit message's first sentence should be capitalized, no period
+
+    It may be followed by a paragraph with a longer explanation. The
+    changelog style entry goes at the end of the message.
+
+    * foo.el (a-function): Terse summary of per-function changes.
+
+
+[bugnote]: https://github.com/capitaomorte/yasnippet#important-note-regarding-bug-reporting