Changes: document incompatibility with <= 15.1 binary signers

Context

Up to v15.1 signature binary raw encoding was Data_encoding.Fixed.bytes because all signatures were of same length. It is now Variable.byte because bls signatures are bigger. But binary encoding of variable bytes is the length + the byte instead of just the bytes. Retro compatibility is broken!

Manually testing the MR

Read the doc.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR