update readme with giphy stuff

[HarHarLinks]

e.g. the feature comparison table but maybe also setup stuff and so on :)

[tulir]

There's no setup stuff for giphy, it should just work

[HarHarLinks]

what about running your own proxy and api key?

[hegdenischay]

That should be specified as giphy_mxc_prefix in the config, but I don't think the python program asks for it anymore.