Consider saying that custom methods **should** use GET or POST.

[rofrankel]

          Let's go back to the Google guidance here.

• The HTTP method must be GET or POST:
  • GET must be used for methods retrieving data or resource state.
  • POST must be used if the method has side effects or mutates resources or data.
• The HTTP URI must use a : character followed by the custom verb (:archive in the above example), and the verb in the URI must match the verb in the name of the RPC.
  • If word separation is required, camelCase must be used.

Originally posted by @mkistler in #103 (comment)