Add CONTRIBUTING.md

[0b10011]

I would recommend adding a CONTRIBUTING.md file per https://github.com/blog/1184-contributing-guidelines

This will ask any user opening a new issue or submitting a pull request to review the contributing guidelines of Brackets. It will help prevent issues such as this.

Some examples

• CodeMirror
• jQuery

Useful information

• Coding standards for project
• Requirements for PRs (eg, all tests must pass, sign CLA, etc)
• Requirements for opening a new issue (eg, make sure it doesn't already exist, look on trello, etc)
• Which features can go in the core, and which should go into extensions

[peterflynn]

Perhaps we should just link to the existing main Brackets guide - https://github.com/adobe/brackets/blob/master/CONTRIBUTING.md