Add contributing.md (#191)

* first cut at contributing.md

* second cut at contributing.md
- Give clearer purpose to contributing instead of a replacement for HACKING.txt
- Add Get Support section
- Rewrite section now named Working on issues
- Rewrite Running Tests section
- Rename Building documentation for a Pylons Project project to Contributing documentation

* Fix typos.

* Update links; unwrap lines
2 files changed