document headers we send to payloads
link to guids
use the headers we actually send not what rails gives us
Merge remote-tracking branch 'origin/master' into webhook-header-docs
stupid underscores
restructure the locations for docs
add some content, still needs something better
thanks for the suggestions @pengwynn
Merge pull request #451 from github/webhook-header-docs

Document headers we send to payloads