Add documentation for getting user context from Coherence sessions

[DavidAntaramian]

@pmarreck provided a code snippet for adding the user context based on a Coherence session. We should add this to documentation (with appropriate credit). This can probably go into the Timber.Contexts.UserContext module documentation.

Link to snippet: https://gist.github.com/pmarreck/4e0803a1a6a319a88b211b9669f38999