Add documentation for multilanguage support

[jamesmaa]

We're getting inbound questions about whether yomitan supports _ language. Let's add a section in the README mentioning multilanguage support and a table indicating which ones are supported and what degree

[brishtibheja]

What do you guys think about changing the description to remove the word Japanese in "Japanese pop-up dictionary browser extension". This is in the top of the main page.

Just my humble opinion, I think rewording sentences like, "Yomitan turns your web browser into a tool for building Japanese language literacy..." might reflect the usefulness of Yomitan for non-Japanese languages better.