Pip usage in documentation not ideal

[damonlynch]

Gidday!

A couple of points:

1. Updating pip when you don't need to is a bad idea, because some (most?) distros patch pip, and upgrading pip from PyPi does not account for their patches. See for instance this from the Python guy at Fedora. Instead I suggest recommending a minimum pip version needed to install pyheif
2. Instead of running pip directly, to my best knowledge python3 -m pip or python -m pip is now recommended as best practice by the pip developers. I can't recall where I read that however. Something like this: python3 -m pip install --user pyheif. AFAIK newer versions of pip don't need --user but older versions do.