Update mamba install instructions

[rbeucher]

Hi,

Thanks for all the work and for providing the package to the community.

We spent quite a bit of time trying to figure out how to install the packages in a mamba environment. I took the liberty to update some of the instructions. This might be useful to others.

• I am using Miniforge. I believe this is the most lightweight option.
• The old version was creating a Python virtual environment, not a conda environment.
• mamba init should be run not just conda init

Romain

[alexisInSAR]

Merged!

Thank you.