Document ToTry timeout importance in README

[nikitapecasa]

This is need to give a clear warning/understanding about default timeout which would be used in case of RebalanceListener1 (implemented in #122 )

• at the moment of writing default timeout for cats effect IO is 1 minute https://github.com/evolution-gaming/cats-helper/blob/3afd4488d76de8873853fc71ce9e43a3505ca491/core/src/main/scala/com/evolutiongaming/catshelper/ToTry.scala#L49
• so only 1 minute is given to complete a single rebalance callback (onPartitionsAssigned/onPartitionsRevoked/onPartitionsLost)
• actual timeout depends on implementation details of underlying effect/cancellation guarantees and the user code itself
• add an example of providing custom ToTry timeout in Readme