add ansible-galaxy to command cheatsheet (#826)

Part of #48

Adds common CLI use for `ansible-galaxy` command.

docs/docsite/rst/command_guide/cheatsheet.rst

@@ -31,3 +31,24 @@ Loads ``my_playbook.yml`` from the current working directory and:
 
 See :ref:`ansible-playbook` for detailed documentation.
 
+
+ansible-galaxy
+==============
+
+Installing a collection:
+
+.. code-block:: bash
+
+   ansible-galaxy collection install mynamespace.mycollection
+
+Downloads ``mynamespace.mycollection`` from the configured Galaxy server (`<galaxy.ansible.com>`_ by default).
+
+
+Listing all installed collections:
+
+.. code-block:: bash
+
+   ansible-galaxy collection list
+
+See :ref:`ansible-galaxy` for detailed documentation.
+