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
should we add `package-i18n` with the title and description in french?