Add documentation about usage as rails server.

[robotex82]

Hi,

I just stumbled on this project. Looks very interesting. Especially the graphQL integration!

To get a feeling about the usage I tried to start a server with a rails app. First i tried with rackup but i failed specifiying the bind parameter.

Then I had a look at how rails chooses a web server. It is pretty simple, so i updated the docs.

I hope this helps others trying to usa agoo with rails!

Thank you!

[ohler55]

Thanks.