Figure out minimal GITHUB TOKEN permission set

[Jörg Thalheim]

We should document what minimal permissions a token needs:

Requests in buildbot-nix:

• GET "https://api.github.com/repos/{owner}/{repo}/hooks?per_page=100"
• GET "https://api.github.com/user/repos?per_page=100"
• POST "https://api.github.com/repos/{owner}/{repo}/hooks"

Requests in buildbot:

GitHubStatusPush:

• POST "https://api.github.com/repos/{owner}/{repo}/statuses"

AvatarGitHub:

• GET "https://api.github.com/users/{username}'
• GET "https://api.github.com/search/users"
• GET "https://api.github.com/search/commits"

GitHubEventHandler:

• GET "https://api.github.com/repos/{repo}/pulls/{number}/files"
• GET "https://api.github.com/repos/{repo}/commits/{sha}"

[Jörg Thalheim]

We have this now documented in our README.