Charter claims to have never been modified

[hober]

At the very top of the charter, it says this:

• Start Date: 2 July 2015
• Last Modified: 2 July 2015

But it's clear from charter.html's revision history that the charter has changed a number of times since then.

Minimally, the "Last Modified" line should be updated when the charter is changed. Ideally, the charter would include links to all previous versions.

[marcoscaceres]

Thanks @hober for spotting that! Dynamically updated the last modified, plus added link to commit history.