[ANNOUNCE] 408k-rc1

[Pyry Kovanen]

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The first release candidate for 408k is out, the highlights are:

* Begin migration of all up-to-date ships to directed messaging.

* Ford lightning allows us to build all marks eagerly, avoiding terrible first-use performance.

* Rip out doccords to improve hoon compiler performance.

For detailed instructions on how to test the release candidate and migrate your desks for 408 compatibility see https://github.com/urbit/urbit/releases/tag/408k-rc1