Add installation instructions for conda forge

[jaimergp]

Will close #6

[raimis]

@jaimergp is this finished?

[jaimergp]

I tested on Linux and it works. It'd be nice to check if MacOS works as expected too:

conda create -n openmm-plumed-test -c conda-forge openmm-plumed
conda activate openmm-plumed-test
python -c "import openmmplumed"  # should return nothing
$CONDA_PREFIX/share/openmm-plumed/tests/TestReferencePlumedForce

[tonigi]

It seems to work on MacOS too :) [Tested on Big Sur x86 according to the instructions. I would be surprised if it's using any acceleration]

[raimis]

I think, we can merge this.

[jaimergp]

@peastman Ready to merge :)