doc: Add a contributing file at the root of repo

Github shows up a `guidelines for contributing` while opening a new pull
request for any repository with a CONTRIBUTING file at the repo
root, which allows for a quick overview to contribute to the
repository.

Currently this file just links to the Submitting Patches and the
documentation start guide.

Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@gmail.com>
Merge pull request #2408 from theanalyst/doc/contributing

doc: Add a contributing file at the root of repo

Reviewed-by: Loic Dachary <loic@dachary.org>