Updated installation instructions to include requirements.txt

doc/installation.rst

@@ -16,6 +16,14 @@ Full installation instructions can be found on the project website.
 
 To run the tests, the `pytest <http://docs.pytest.org/en/latest/>`_ module must be installed.
 
+Pip
+^^^
+
+To install all dependencies automatically, using Pip, a *requirements.txt* file is provided.
+This can be invoked as follows::
+
+    pip install -r requirements.txt
+
 PyPy
 ^^^^