document the github access token scope
document the github access token scope

[finishes #102513822]

Signed-off-by: Evan Short <5b1db704470d35a058f1f4a9e1a5a172a6ea69d3@pivotal.io>

apcera/github-release-resource,jtarchie/github-release-resource,zachgersh/github-release-resource,zachgersh/github-release-resource,concourse/github-release-resource,jtarchie/github-release-resource,apcera/github-release-resource