README needs update for bitpacks, new test modules

[ToxicFrog]

The README basically hasn't been updated since work on the new parser started. It needs it before this module can be considered release-ready again.

In particular:

• ()-groups are not mentioned
• post-repeat constructs are still listed
• the section on bitpack format needs fleshing out, in particular the way it interacts with the endianness setting
• how to install and load the module should be documented
• test suites should be documented

It may also be worthwhile to reformat it as either ReStructuredText or GitHub Flavoured Markdown.

[ToxicFrog]

As of 5e602282d79e3405e018f8d4e76e87a03c0064ef the README has been all but completely rewritten.