Format HACKING as markdown.
Rename HACKING to clarify that it is a markdown document.
Document how to run pgtap tests.
Set up pgtap extension in create_test_db.sh.
Use correct schema in CREATE EXTENSION pgtap call.
Merge branch 'pgtap-instructions' into beta

* pgtap-instructions:
  Use correct schema in CREATE EXTENSION pgtap call.
  Set up pgtap extension in create_test_db.sh.
  Document how to run pgtap tests.
  Rename HACKING to clarify that it is a markdown document.
  Format HACKING as markdown.