Create the reviewers guide base document
What does this MR do?
Adds a base of a guidance document for GitLab Runner reviewers. Inspired by the discussion at https://gitlab.slack.com/archives/CBQ76ND6W/p1551803645032500.
Adds a base of a guidance document for GitLab Runner reviewers. Inspired by the discussion at https://gitlab.slack.com/archives/CBQ76ND6W/p1551803645032500.