Add response path to flow diagram

So indicate that we're handling both sides.
Reference stunnel in README about mocks

We now use stunnel to wrap the request to our backend in TLS.
Add flow diagram for mock VM

To show how the components fit together.
Fix comment about adding mock configuration

All existing tests should pass. But you might need to change the
configuration when adding new tests.
Document that helpers should be used where poss

These do some of the work for you such as error checking.
Merge pull request #85 from alphagov/improve_readme

Improve README