+ link to http://brackets.io/contribute/#get-coding

Found a useful resource for new hackers.

Not ready to merge; wondering if the next 2 paragraphs should be moved to CONTRIBUTING.md or the WIKI please?
```
The text editor inside Brackets is based on 
[CodeMirror](http://github.com/marijnh/CodeMirror)&mdash;thanks to Marijn for
taking our pull requests, implementing feature requests and fixing bugs! See 
[Notes on CodeMirror](https://github.com/adobe/brackets/wiki/Notes-on-CodeMirror)
for info on how we're using CodeMirror.

Although Brackets is built in HTML/CSS/JS, it currently runs as a desktop 
application in a thin native shell, so that it can access your local files.
(If you just try to open the index.html file in a browser, it won't work yet.)
The native shell for Brackets lives in a separate repo, 
[adobe/brackets-shell](https://github.com/adobe/brackets-shell/).
```