Add requirements.txt and instructions for installation with pip

[willhansen]

I wanted to run the latest master branch of CQ-editor, and I didn't want to deal with conda, so I tried out pip3 install -e ., then just installed dependencies as error messages said they were missing, and it worked.

Cadquery now supports a non-conda setup, and it doesn't seem like too much work, so it might be worth supporting the workflow.