Add a Compatibility Policy Document

[rbair23]

Problem

It has been requested by members of the community that we create an official compatibility policy document detailing the position of the TSC regarding compatibility. This document must be reviewed and voted on by the TSC.

Solution

A strong bias towards compatibility, with carve-outs for immature projects and planned deprecation and security needs.

Alternatives

No response

[rbair23]

Interesting contemporaneous example of another chain (Ethereum) with a backwards incompatible change: https://eips.ethereum.org/EIPS/eip-7825