reconcile readme install instructions

[shortwavedave]

While running through the readme instructions, this command: pip install -r ".[test]" produces this error: ERROR: Could not open requirements file: [Errno 2] No such file or directory: '.[test]'

This pull request updates the readme to match what the Dockerfile prescribes: pip install -e .

[carlodef]

Hi @shortwavedave, there is a typo in your command. The readme says pip install -e ".[test]", with -e, not -r.

The command is tested in github actions (see https://github.com/centreborelli/s2p/blob/master/.github/workflows/tests.yml#L18) and works fine.