Document that OkHttpClient is thread-safe

While investigating a race-condition, I was momentarily worried when I
couldn't find any documentation actually stating that OkHttpClient is
thread-safe, apart from this Google+ post:

https://plus.google.com/118239425803358296962/posts/5nzAvPaitHu

...if I'm not the only person asking this question, probably best to
document it.
Doc: OkHttpClient need not /always/ be a singleton
Doc: OkHttpClient should be treated as immutable once shared

...given that requirement, it's safe for many threads to call the
open() method concurrently to open their own connections.

https://github.com/square/okhttp/pull/634#discussion_r10487815
Merge pull request #634 from rtyley/okhttpclient-is-threadsafe

Document that OkHttpClient is thread-safe