Add internationalization documentation to README

[samajammin]

We should explain how internationalization works on this project:

1. How to get involved as a translator (e.g. by linking to https://crowdin.com/project/launchpad-translation & explaining how it works)
2. How we configured the codebase for internationalization (i.e. using react-intl, with extract & compile scripts to play nice with Crowdin & React) so devs know how to add new languages (e.g. see #411), make changes & support content updates (like typo fixes, as these require updates to the json hashes - e.g. see #407)

[samajammin]

FYI @wackerow - perhaps a good one to pair on with @corwintines or @pettinarip next time we need to add a language 😄