Update CHANGES file for 0.21.0 release
Update CHANGES file for 0.21.0 release

Also updated changes for 0.20.1, and added a small note about changes
since 0.12.0.

tb00/btcd,btcsuite/btcd,davecgh/btcd,ltcsuite/ltcd,tb00/btcd,davecgh/btcd,ltcsuite/ltcd,btcsuite/btcd