Enhancing conda environment set up instructions

[matt-graham]

Submitting as part of JOSS review openjournals/joss-reviews#3397

In the 'Install with conda' instructions in the README file the conda install pip line is (I think) unnecessary as I think installing python in a conda environment will also always install pip. All of the requirements also seem to available as conda packages if the conda-forge channel is used, so to avoid mixing conda and pip an alternative to create the environment and install all of the dependencies would be

conda create --name pyei --channel conda-forge python=3.8 --file requirements.txt --file requirements-dev.txt