Document if the classes are threadsafe.

[DavidBiesack]

Can a Parser or HtmlRenderer be used in a threaded environment - shared across threads?

[robinst]

Yes it can! It takes care to use new state for each parse/render call. Although I should probably add tests for that and document it properly.

[DavidBiesack]

Thanks, @robinst -- as suggested, please add a note about this in the README. Thanks!

[robinst]

Added tests and a section to the README now, thanks for raising this!