Add instruction for translators

[szabgab]

I would like to create a transaltion of the game and send a PR. It would be nice if the README had the instructions.
I came up with this:

How to translate

To translate the interface create the appropriate file in the src/i18n/messages/ directory translating the entries from src/i18n/messages/en-CA.ts and add the new entry to src/i18n/messages/index.ts

To allow the user to select a language add the entry to the langMap in in src/components/LanguagePicker.tsx.

To be able to use the country names in the specific language add the translations of the country-names to src/data/country_data.json

Is this enough?