Remove the old to-do list.

It's out of date (from before launch) and virtually invisible.
Remove "work in progress" caveat.
Summarise what the content API is used for.
Update the command used to run the test suite.

We shouldn't be relying on `govuk_setenv` in development, and our code
doesn't depend on the environment variables it sets since Plek 1.1.0.
Merge pull request #83 from alphagov/update-readme

Update README