requirements.txt: document that it contains python libraries

[miri64]

See #83 (comment) ;-)

[benpicco]

I guess people who used Docker before might have already known 😅
But a little comment sure can't hurt for those who haven't.

[miri64]

I guess people who used Docker before might have already known

It's not docker, it's our Dockerfile ;-). Calling the requirements file requirements.txt is a python convention, so if you never wrote a python package, the confusion is understandable ;-).

See also #56 for historical reference ;-)