Add example of generics use to the docs

[phadej]

Explain:

fieldHref_
    :: ( HasLink endpoint, IsElem endpoint (ToServantApi routes)
       , GenericServant routes AsApi
       )
    => (routes AsApi -> endpoint)
    -> MkLink endpoint Attribute
fieldHref_ = fieldLink' linkAbsHref_

[rashadg1030]

Is the code for this already available? Or is this something that should be added? I can work on this since I have a library that depends on this sort of functionality. The more general linkAttribute one at least. This would be nice and allow me to push this out of my library, upstream.

[rashadg1030]

Wait nevermind this already included in the latest version but it would be nice to have this for other/custom attributes as well, like you mentioned in #7.