Updates to documentation:
- CONTRIBUTING.md: big revamp. Broaden scope; include bug & pull request
info at top (since it's linked from those creation pages); clean up wording;
remove some info that's redundant with other pages & link to them instead
- README.md: soften "it's early" messaging a tad; emphasize CONTRIBUTING.md
more; revamp bug-filing section; add small "What's Brackets working on
next?" section
- nls/README.md: tighten wording; update list of languages
- Getting Started: fix typos
- LanguageManager: fix incorrect statement about extensibility
Code review & other tweaks to .md cleanups:
- add top heading to CONTRIBUTE
- reorder some bullet lists
- fix missing word
- further softer the 'try out Brackets' wording
- add another link to How to Hack on Brackets
- make "Ways to Contribute" list more consistently bolded
Merge pull request #3846 from adobe/pflynn/md-docs-cleanup

Updates to bigger documentation pages