Add internationalization documentation to README

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)