Merge branch '11-contributing' into 'master'

Add contributing guidance. Fixes #11

Closes #11

See merge request !1

CONTRIBUTING.md

+# Guidelines for Contributing
+
+Contributions to this project are very welcome. 
+Changes should be submitted by merge request from a new branch to `master`.
+Merge requests should be reviewed by at least one Maintainer before merging.
+
+See `README.md` for information on the structure of the repository.
\ No newline at end of file