Documentation: add the Heads project to the list of distributions

As this is a unique, actively maintained project, we should probably
point there too. The text is just copied parts of the http://osresearch.net/
website.

Change-Id: Ib2a8e4b28bc94c5dc6a1ae9388f96ad2c502ccab
Signed-off-by: Martin Kepplinger <martink@posteo.de>
Reviewed-on: https://review.coreboot.org/c/31257
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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