Pool size config documentation

[PragTob]

Implements #395 :)

Not sure if this is the best way, but I added the information I wish I'd seen yesterday and also implements a small pointer from the README. Keep me honest on that, esp. my meta note on configuration :)

Thanks! :green_heart:

[PragTob]

@wojtekmach anything I can do to help move this along?

[PragTob]

@wojtekmach looking good! I reworded it slightly in a588da2 as like for example didn't read nicely to me there :)

[wojtekmach]

Thank you!