Update README: "prequ update", "requirements.pre"
Update README: "prequ update", "requirements.pre"

Now "prequ update" is considered the main interface for generating the
`requirements*.txt` files.  Amend README to adhere to that.  Also add a
more detailed example of the new `requirements.pre` file.

suutari/prequ,suutari-ai/prequ,suutari/prequ