Installation docs assume conda-forge channel already in conda config

[mcs07]

It's really great that you have a package on conda-forge for easy installation!

This might be a bit of an edge case, but if users install mamba into an existing conda install (e.g. from Miniconda or Anaconda), they won't automatically get the conda-forge channel pre-configured (like it would be from a mambaforge install). In this case, the installation instructions as written won't work.

It could be worth extending the existing "Note" about mamba to mention this, for example:

If you installed mamba into an existing conda installation, also make sure that the conda-forge channel is configured by running conda config --add channels conda-forge.

Link to JOSS review: openjournals/joss-reviews#3951

[dominiquesydow]

Hi @mcs07,

Thanks a lot for this suggestion! I have added the suggested note to our documentation.
Would you like to take a look before I merge?
#130