Explicitly state max_connections and timeout defaults in docs

[sbinlondon]

There's no explicitly stated defaults (that I can find) for max_connections and timeout.

The README examples, if they are examples of the defaults, say 100 for max_connections but then in hackney.app.src it seems to be 50? I went with that one but if it is something else, please let me know and happy to update my PR.

Either way it would be helpful to have very clearly stated defaults.

[sbinlondon]

@benoitc let me know if that new formatting is ok or if you want any more changes.