Add note to doc about conda installation of esmpy

[aulemahal]

With esmpy 8.4 the conda recipe has changed. There a library file that now gets linked through an environment variable. When installing with conda, this environment variable is set upon activation of the virtual env, and NOT when the installer runs.

Thus, one can't do:

conda activate myenv
conda install esmpy
python -c "import esmpy"

One must rather do:

conda activate myenv
conda install esmpy
conda deactivate
conda activate myenv
python -c "import esmpy"

This is issue : conda-forge/esmf-feedstock#91.

A note to the documentation should be added.