adding missing friends.getRichPresence() function.

[zarutian]

I was reading the readthedocs documentation and found that the friends.getRichPresence(steamId, key) function was missing.

I have not tested this as I do not currently have access to a computer where I can put up the build environment required. However this should work as this was rather easy to add.

(If there is a GitHub Actions build test runner then it should catch any errors.)