Improve LocalizationExample extension & nls README instructions

LocalizationExample :

- Fix some incorrect/unnecessary code
- Add explanatory notes to more files
- Clarify which code in main.js applies to which string-usage approach
- Rework README so it more clearly applies to extensions (linking to wiki & nls README for info on other topics)

nls README:

- Move 'how to add new locale' & 'translate Getting Started' sections down from the top, since they're not the most common tasks.
- Explain how to use SHA to diff English string changes (which is the point of the SHA, after all)
- Clearly separate info on local Git workflows vs. using GitHub online
- Remove info on adding a new locale using GitHub online - it omitted all the other files that need to get updated (and multi-file updates are tricky on GH online anyway)
- Add more info on using Git locally: how to get the SHA & how to update existing translations
- Smaller updates: clean up wording, eliminate unnecessary details, add clarifications
I do not think that this section should be removed. There are already people attempting to add/update translations directly on the web site, so this explains the process. These are people that do know know git, and do not have the desire to learn git, so simply telling them to "contact the Brackets team" does not help them or us. It's much easier to have this explanation for them to read.