Added PostgreSQL installation. (#1846)

PostgreSQL is needed to install dev-requirements.txt. Otherwise psycopg2 will fail.
PostgreSQL needs to run to be able to execute ./manage.py migrate

Author: johannes-pirmann

docs/source/install.rst

@@ -3,8 +3,11 @@ Installing
 
 Manual setup
 ------------
+First, install PostgreSQL_ on your machine and run it.
 
-First, clone the repository::
+.. _PostgreSQL: https://www.postgresql.org/download/
+
+Then clone the repository::
 
     $ git clone git://github.com/python/pythondotorg.git