use new goreleaser scoop config

Merge pull request cli#289 from atomiyama/misc/modify_installation

ADD installation from source section to README.

put gh.rb in the right place

Merge pull request cli#298 from cli/scoop

scoop config for gh

ok right duh

try and change bucket name

s/-/_/

add scoop config to goreleaser

Merge pull request cli#272 from cli/windows-urls

Fix `pr create` preview in browser flow for Windows

Consistently use `checkout@v2`