Deprecating functionality policy

[knmcguire]

In bitcraze/crazyflie-firmware#849 we propose to deprecate a crtp packet, but how do we want to handle how to deprecate these in the first place?

• The period of time. Do we mention a date when it will be deprecated
• Indicating the alternative if there is one
• Handling warnings. Do we want write deprecation warning in the console?

Anyway, this should be a discussion at one point how to handle this.