Add example showing how to access shared state from middleware in 1.0.0-alpha.6

[ctsrc]

Accessing shared state in handlers is demonstrated in https://github.com/actix/examples/blob/815e34f32ded77f73145fb0373a060b22636f689/state/src/main.rs

And with the way middleware worked previously, you could access shared state in the middleware as such: https://stackoverflow.com/questions/50428385/application-state-access-from-an-actix-web-application-middleware/50428541#50428541

But with the way middleware works now, as demonstrated in https://github.com/actix/examples/blob/09b0188ff99e872898793bd815382a4105219b77/middleware/src/redirect.rs. it is not apparent how one would access shared state from middleware.

Adding an example showing how to access shared state from middleware in 1.0.0-alpha.6 would be immensly helpful.

[ctsrc]

Got an answer to this on Reddit from one of the people involved with actix. It's quite simple so no example for this is needed after all I think.

https://docs.rs/actix-web/1.0.0-alpha.6/actix_web/dev/struct.ServiceRequest.html#method.app_data