Add an 'About this document' section.

This commit adds an 'About this document' section at the foot of the
document. It is intended to address some of the points raised in

It adds a short paragraph on the origin of the document, and then
outlines how subsequent, and future changes to it can be made. I've
hand-waved a bit on how proposed changes are incorporated into the
document, but based it on the closed issues in the github repo, that is,
the discussion around any changes is open and in the public, and when
consensus is reached the changes are merged in.

On the issue of volunteering, to address primarily how named people in
the document are 'appointed', I've added a short paragraph on
volunteering, which encourages the reader to contact @hlame in the first
instance.
Create a separate section for the volunteer team

The volunteer team will be referred to in other places in the document,
so is moved into its own section.
Add a 3rd option for suggesting changes

Lower the barrier to entry for making changes to the document by
suggesting that the reader contacts Murray or one of the volunteer team
privately.
Clarify the provenance of the document

Mention that Tom and Aanand were co-authors of the original draft, and
link to the github contributors page.
Move information on joining the volunteer team

The information on how to join the volunteer team makes more sense in
the volunteer team section.
fixup! More information on joining the volunteer team

Re-word the joining instructions.
Merge pull request #25 from chrislo/address-issue-21

Add an 'About this document' section.