Document backwards compatibility expectations

[urfeex]

Add a statement on our backwards compatibility structure. Things that come to my mind immediately:

• API stability inside major version
• Not necessarily ABI stability guaranteed
• Compatibility to robots as declared in https://github.com/UniversalRobots/Universal_Robots_Client_Library/blob/master/doc/polyscope_compatibility.rst
• Custom communication protocol stability (Reverse interface, Trajectory Point interface, etc) not necessarily given. When script is extended, users have to make sure, they follow protocol changes when updating the library.