Add windows installation note (#945)

* Add windows install note

* Better formatting (test)

* Update docs/getting_started/installation/installation.rst

Co-authored-by: Co Quach <43968221+daico007@users.noreply.github.com>

Author: umesh-timalsina

docs/getting_started/installation/installation.rst

@@ -17,6 +17,11 @@ after which you can simply install without specifying the channels::
 .. note::
     The order in which channels are added matters: ``conda-forge`` should be the highest priority as a result of being added last. In your ``.condarc`` file, it should be listed first.
 
+.. note::
+    Because ``packmol`` binaries are unavailable for windows from ``conda-forge`` channel, to use mbuild with conda in a Windows system requires the ``omnia`` channel. Use the following command to use ``mbuild`` with conda in a Windows system::
+
+        $ conda install -c conda-forge -c omnia mbuild
+
 .. note::
     The `MDTraj website <http://mdtraj.org/1.9.3/new_to_python.html>`_ makes a
     nice case for using Python and in particular the