TimeZoneEngine Thread Saftey after Initialization

[ivanthewebber]

Is the TimeZoneEngine thread safe for concurrent queries after initialization?

Poking at the source code I think the answer is yes, but I think it makes sense to confirm and document it.

[RomanIakovlev]

Yeah I think it should be thread-safe, since most of the data is read-only after initialization, and the only mutable variables are method parameters and are therefore thread bound. If you want to make a PR to the README with this information, I'd gladly accept it.