add pip to requirements.txt code example

Example 1: pip freeze requirements.txt

$ env1/bin/pip freeze > requirements.txt
$ env2/bin/pip install -r requirements.txt

Example 2: python requirements.txt

pip freeze #view requirements to be created (best done in virtual env)
pip freeze > requirements.txt #create requirements.txt (best done in virtual env)
pip install -r requirements.txt #install requirements.txt (best done in virtual env)