Add "Manually managing cache usage" to README
Add "Manually managing cache usage" to README

* Small amount of documentation regarding manual
cache disc usage
chrisbtreats/SPTPersistentCache,iOSCowboy/SPTPersistentCache,iOSCowboy/SPTPersistentCache,iOSCowboy/SPTPersistentCache,FootballAddicts/SPTPersistentCache,FootballAddicts/SPTPersistentCache,chrisbtreats/SPTPersistentCache,FootballAddicts/SPTPersistentCache,spotify/SPTPersistentCache,chrisbtreats/SPTPersistentCache,FootballAddicts/SPTPersistentCache,spotify/SPTPersistentCache,spotify/SPTPersistentCache,chrisbtreats/SPTPersistentCache