multi-wc-format: release notes

[Daniel Shahaf <d....@daniel.shahaf.name>]

Shall we write release notes for multi-wc-format now?  That
branch/feature seems merged and finished, other than the release notes
not being updated yet.  And it's better to write them now while it's
fresh in our memories, and so the notes will be available to anyone who
might try trunk@HEAD builds.

There's a start here:

https://subversion-staging.apache.org/docs/release-notes/1.15#wc-upgrade

Any volunteers?

Cheers,

Daniel