fix(*): Document using gpg detached signatures instead of DIGESTS

As of release 298 we generate .sig files which are much easier to deal
with than the oddball DIGESTS format.
Merge pull request #183 from marineam/sig

fix(*): Document using gpg detached signatures instead of DIGESTS
Update Vultr instructions.

Vultr now supports automatic iPXE chaining, so there is no need to play with the console any more.

Also, add an information about installation to the disk - as the lack of it made me confused.
Merge pull request #187 from LEW21/patch-1

Update Vultr instructions.
chore(vultr): clean up vultr doc
Merge pull request #188 from jonboulle/master

chore(vultr): clean up vultr doc
feat(running-coreos): add channel instructions
fix(ec2,gce,rax): include additional tabs for switching channels
fix(bare-metal): improve channel selection info