Update devguide/provide write up on how to sign CLA using cla-assistant.
It should cover the scenario for when CLA is needed on new pull requests, as well as the scenario when someone wants to sign the CLA in advance.
GitMate.io thinks possibly related issues are https://github.com/python/devguide/issues/6 (Update devguide style to use a similar theme as Docs), https://github.com/python/devguide/issues/116 (Update Devguide about building documentation.), https://github.com/python/devguide/issues/300 (devguide was last updated at None.), https://github.com/python/devguide/issues/133 (Update Devguide about pull request commit documentation.), and https://github.com/python/devguide/issues/308 (Update the devguide with the discussion on bug triage).