Document best practices for interfaces and interface libraries

There are common problems faced by many interfaces, like how to add features to an interface in a backwards compatible way, often by extending the meaning of an existing key, or adding a new optional key. We should document best practices for interface maintainers and interface library authors for dealing with common challenges like this.

Some questions around this are captured in https://github.com/canonical/charm-relation-interfaces/issues/14

Start by reading the linked charm-relation-interfaces issue #14 and then locate the repository’s existing documentation entry points for interface libraries. Done means documenting guidance for backward-compatible interface changes, including extending existing keys and adding optional keys, along with the other common challenges identified in the issue.

Written by the indexing model from the issue text.