add url to docs to github project details

[wbolster]

it would be nice if the github project points to

https://pysaml2.readthedocs.io/

to make it more discoverable. it would appear at the top of the github page.

[wbolster]

heh, thanks, but i actually referred to the project description and url on github itself, see e.g. https://github.com/wbolster/jsonlines that looks like this:

[c00kiemon5ter]

I thought of that too, but I have no such rights.
Maybe @rohe can do that.