Add documentation for online region

[geoffrey-wu]

The online "region" is defined as 0P, but there is no mention of this in the map/regions.js file or the README. It would be useful to include this somewhere so people don't have to guess / snoop around the source code.

[hftf]

Thanks for the report! Iirc, 0P is for online playtests, and 0L is for online (not playtest). Would you like to make a PR to add some documentation?

[hftf]

I forgot: There are also tooltips in the main table if you hover over a region, though it doesn't count much for documentation. (See also #5 for an idea of a graphical tooltip, which already has at least an existing janky implementation.)

[hftf]

Addressed in 4ccacc0.