Updates to bigger documentation pages

- _CONTRIBUTING.md_: big revamp. Broaden scope; put bug & pull request info at top (since it's linked the 'New Issue' & 'Create pull request' 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

@njx hoping you might have time to review this.  CC @adrocknaphobia @pthiess @gruehle in case you guys have feedback too.