Add documentation on how service fees are handled

[uditvira]

@Art3miX to document:

• Fees are defined per service
• Accounts must send fees with registration message to service manager
• Service manager passes these on to service during registration
• Service verifies fees and returns them to service manager
• Service manager collects fees
• Registration and resume (after paused by the service itself, note that manual pauses are not subject to resume fees) follows the same process
• Admins of the service manager may execute withdraw { "denom" : string } and the tokens go to the admin