Installation instructions out of date

[mslinn]

requirements.txt has been moved to requirements/main.txt, so the PyPi instructions are now out of date.

The correct incantation is now:

pip install -r requirements/main.txt

[rufuspollock]

@mslinn thanks for catching - do you want to submit a PR?

[mslinn]

I would have, but I did not know which repo needed the modification.

[rufuspollock]

@mslinn this repo here ... in the docs section i believe 😄 https://github.com/datopian/giftless/tree/main/docs