adds CONTRIBUTING.md

1st draft commit of @janl's docs contributions content
(http://docs.couchdb.org/en/latest/contributing.html)
as a github project contribution guideline, as per
(https://github.com/blog/1184-contributing-guidelines)

---
:+1: 
The **upside** to maintaining a [bonafide CONTRIBUTING policy file](https://github.com/blog/1184-contributing-guidelines) here in the root of the project tree, is that it becomes easily discoverable by folks who arrive at `apache/couchdb` project on github to open issues and submit pull requests... the policy is revealed thusly above both the "create new issue" and "create new pull request" forms:
![Screen Shot 2013-02-21 at 10 56 57 AM](https://f.cloud.github.com/assets/194904/181156/bc9c6332-7c47-11e2-9f35-fa319020e5fe.png)

:-1: 
The **downside** is dual-maintenance - keeping this `CONTRIBUTING.md` policy file in sync with the same content in the [`share/doc/src/contributing.rst`](https://github.com/apache/couchdb/blob/master/share/doc/src/contributing.rst) file (plus whereever the source code contribution guidelines are?).

Unless someone knows of a clever way to reconcile that to NOT be dual-maintenance.