Update devguide/provide write up on how to sign CLA using cla-assistant.

[Mariatta]

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.

[Mariatta]

GitMate.io thinks possibly related issues are python/devguide#6 (Update devguide style to use a similar theme as Docs), python/devguide#116 (Update Devguide about building documentation.), python/devguide#300 (devguide was last updated at None.), python/devguide#133 (Update Devguide about pull request commit documentation.), and python/devguide#308 (Update the devguide with the discussion on bug triage).