docs: installation: Add link to Quickstart after install

Once Flask is installed many users will want to proceed to the next section
in the docs (Quickstart).  Add a link to the end of the Install section for
this.

docs/installation.rst

@@ -142,6 +142,8 @@ update the code from the master branch:
 
     pip install -U https://github.com/pallets/flask/archive/master.tar.gz
 
+Once you've installed Flask you can continue to :ref:`quickstart`.
+
 .. _install-install-virtualenv:
 
 Install virtualenv