add CONTRIBUTING (easy commit prefix guidelines ;) )

[ci skip]

CONTRIBUTING.markdown

@@ -0,0 +1,49 @@
+
+Thanks for contributing to CIEL. We hope it is useful to you and will
+be now more useful to everybody.
+
+Please follow these short guidelines. They'll help the maintainer(s)
+craft release notes and they make for a clearer commits log, *IMO*. Thank you!
+
+## Commit messages
+
+Please say at the beginning what your commit is changing: is it about
+dependencies? The Makefile, the .asd? The terminal/readline interface?
+
+` README:` for the README
+- `docs:` is for documentation
+- `deps:` for the Lisp dependencies (be more explicit for system-wide dependencies)
+- `CI:`
+- `asd:`
+- `terminal`: for the terminal REPL
+
+Example:
+
+> deps: libmagic-dev is no more required
+
+If your change is about a domain, you can say it up front too. For example:
+
+> database: mention the need of db drivers for binaries
+
+If your change is adding or removing something, you can say this action up front.
+
+If your change is wider or doesn't fit here, don't think harder, just contribute. Thanks.
+
+
+### Minor commit messages
+
+I like to see the `(minor)` mention when the change is really trivial
+and not worth looking at. Likewise, we can grep-it out from the
+release notes.
+
+For example:
+
+> (minor) add site icon
+
+> (minor) make run typo
+
+## Avoid small and useless commits, squash them
+
+Please avoid small commits that say "fix" "fix" and again
+"fix". Squash them into one with a good commit message (see above),
+thank you.