Add explanation of pip freeze and shell redirection.

Lots of our students got the impression that "pip freeze > filename" was
the standard way of creating a file from the shell, which caused some
really confusing problems later on!
Make creation of "runtime.txt" more explicit.

We had a surprising number of students who created runtime.txt with the
`pip freeze > runtime.txt` method, and then added a Python version at
the end. This caused endless confusion when Heroku told them that
(Django==1.7.1
dj-database-url==0.3.0
waitress==0.8.9
whitenoise==1.0.5
psycopg2==2.5.3
python-3.4.2) was not a runtime supported by the cedar-14 stack - at one
point we had five coaches standing around one poor student trying to
work out what had gone wrong!

I've tried to make it clearer that

 - you create runtime.txt using your editor
 - only the python version should be included in the file

I've also removed the word "simply" from these instructions in line with
the Coaching Guide.
Merge pull request #205 from pozorvlak/pip_freeze

Pip freeze