[Sessions] Audit logging and update README with strategy

[samedson]

Updated README: https://github.com/firebase/firebase-ios-sdk/blob/bc76232a4e0bf58daf00b7c3c4640805722d1477/FirebaseSessions/README.md

Example with -FIRDebugEnabled

[FirebaseSessions][I-SES000000] Version 10.4.0
[FirebaseSessions][I-SES000000] [Settings] Cache is not expired, no fetch will be made.
[FirebaseSessions][I-SES000000] Initiated new Session with ID: 3152c81f3bad4b518c2a1e0b4459509f
[FirebaseSessions][I-SES000000] Logging events using fast QOS due to running on a simulator
[FirebaseSessions][I-SES000000] Printing Session Event due to "-FIRSessionsDebugEvents" command line argument
[FirebaseSessions][I-SES000000] Successfully logged Session Start event to GoogleDataTransport

#no-changelog