CONTRIBUTING: Switching pip install to use local install

[rm-star]

Problem

The current developer install is using
pip setup.py
which creates a distributable installation. If a developer makes changes
to code then they essentailly need to build and reinstall the package

Solution

If pip install -e . is used then the local site packages will
create a symbolic link which means developers can make cahnges locally
and they will automatically picked up.
It also has the bonus of installing dependencies automatically

A good case for this install format is made here https://stackoverflow.com/questions/42609943/what-is-the-use-case-for-pip-install-e

I am able to test this on linux but not Mac or Windows, though python should
operate in the same way where ever it runs

[rm-star]

I should also say that the -e in the install command means that it is an editable install. I think that helps explain it a bit better