[doc] Add contributing guidelines.

CONTRIBUTING.md

+## Contributing to jsCoq
+
+Thanks for willing to contribute to jsCoq! Contributing is as easy as
+opening a pull request, issue, or dropping by the Gitter channel and
+talking to us.
+
+### Coding style
+
+Nothing very special has to be kept in mind, we follow standard OCaml
+practice, with `ocp-indent` and `ocamlformat` indentation guidelines,
+but we are liberal in some places, in particular with regards to
+intra-line indentation. We compile with a very strict set of warnings.
+
+### Review and merge guidelines
+
+We recommend that most non-trivial changes take place using pull
+requests. Any contributor can merge a pull request [including their
+own] provide the pull request:
+
+- has updated the `CHANGES.md` file,
+- passes Travis CI,
+- has at least one approving review from other contributor.
+
+Exceptions to this rule is when a review cannot happen due to lack of
+a second contributor with that particular expertise.
+
+We prefer GPG signed commits as well as `Signed-off-by` commits.
+
+### Commit tag conventions [work in progress]:
+
+We have some [soft] commit tag conventions:
+
+- [jscoq]: ML/Coq interface
+- [ui]: Html/Css commit
+- [cm]: CodeMirror provider
+- [libs]: Coq Library support and format
+- [doc]: Documentation
+- [addons]: Addons support
+- [build]: Build system
+- [feature]: Adding a new feature
+- [bugfix]: Bug fix
+- [refactor]: Refactoring commit (no functional change intended)
+- [ci] / [travis]: Continuous integration
+- [test]: Adding or modifying a test
+- [misc]: Miscellanenous small things

README.md

@@ -372,16 +372,6 @@ internal data structures, but this effort has been split to an
 independent development. See https://github.com/ejgallego/coq-serapi
 for more information.
 
-## Commit tag conventions [work in progress]:
-
-- [jscoq]: ML/Coq interface.
-- [ui]: Html/Css commit
-- [cm]: CodeMirror provider.
-- [libs]: Coq Library support and format.
-- [doc]: Documentation.
-- [addons]: Addons support.
-- [build]: Build system.
-
 ## Documents
 
 See the `etc/notes/` directory for some random notes about the project.