Adds pipenv shell to docs

It might be confusing for beginners following the docs if the `pipenv shell` is not there

{{cookiecutter.app_name}}/README.rst

@@ -14,6 +14,7 @@ Run the following commands to bootstrap your environment ::
     cd {{cookiecutter.app_name}}
     {%- if cookiecutter.use_pipenv == "yes" %}
     pipenv install --dev
+    pipenv shell
     {%- else %}
     pip install -r requirements/dev.txt
     {%- endif %}
@@ -191,4 +192,4 @@ Deployment by using `Heroku CLI <https://devcenter.heroku.com/articles/heroku-cl
 
     git push heroku master
 
-{%- endif %}
+{%- endif %}