Optional dependencies in README (#330)

Author: itrharrison

README.rst

@@ -56,6 +56,14 @@ The SkyPy library can then be imported from python:
     >>> import skypy
     >>> help(skypy)
 
+SkyPy has a number of optional dependencies which can be installed separately.
+One of these is `skypy-data`_ which contains data such as photometric bandpasses
+required for some calculations in SkyPy. This can be installed with:
+
+.. code:: bash
+    
+    pip install skypy-data@https://github.com/skypyproject/skypy-data/archive/master.tar.gz
+
 SkyPy also has a driver script that can run simulation pipelines from the
 command line. The `skypyproject/examples`_ repository contains sample
 configuration files that you can clone and run:
@@ -72,6 +80,7 @@ configuration files that you can clone and run:
 .. _conda: https://docs.conda.io/en/latest/
 .. _pytest: https://docs.pytest.org/en/stable/
 .. _skypyproject/examples: https://github.com/skypyproject/examples
+.. _skypy-data: https://github.com/skypyproject/skypy-data
 
 
 Contributing