Document how to add context to every log line of a request

[PetrHeinz]

The example project's README only showed around_action for adding context to a request's logs, so readers of #28 could not get the user onto the Started GET … and Completed 200 OK … lines: those are logged by the HTTPEvents Rack middleware before the controller runs. The gem has always had the right hook for this, Logtail::Integrations::Rack::UserContext.custom_user_hash, but no README mentioned it.

The "Context" section of example-project/README.md now explains:

• that Devise (any Warden strategy) and Clearance users are picked up automatically, by a middleware that runs before the request line is logged;
• custom_user_hash: a lambda that finds the user in the Rack env and returns a hash, logged as context.user on every line of the request, including the request and response lines;
• a custom middleware wrapping the request in Logtail.with_context for any other per-request context, inserted before Logtail::Integrations::Rack::HTTPEvents from an initializer;
• around_action stays, as the option when the user is only known inside the controller, with its limit stated.

README.md links to that section.

Both snippets were run as written on the example project (Rails 8.1.4, Ruby 3.3.4, logtail-rails 0.2.14, logtail-rack 0.2.8, logtail 0.1.20) against a local sink: the lambda's hash and the middleware's tenant key arrive on the Started, Processing, in-action, Rendered and Completed rows; the around_action context arrives on the in-action and Rendered rows only.

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