Correct certificate generation information in README

There were a few minor errors in the README file:

  1. The .crt file and not the .pem file should be uploaded to Betfair
  2. The link to the page to upload the .crt file was incorrect.
  3. Several files are generated by `invoke ssl` not just `betfair.pem`

This also includes some minor re-wording and re-formatting to make
the instructions clearer.

This fixes the following issue: https://github.com/jmcarp/betfair.py/issues/6