Rewrite of CONTRIBUTING.md for better contributions

This rewrite of ```CONTRIBUTING.md``` should make it absolutely clear what we expect from contributors. Hopefully, this document will help contributors make high-quality pull requests and bug reports. :)

Verification steps:

- [ ] View the rendered document [here](https://github.com/wvu-r7/metasploit-framework/blob/feature/contributing/CONTRIBUTING.md)
- [ ] Make comments on the diff [here](https://github.com/rapid7/metasploit-framework/pull/3063/files)