Consider adding front matter to describe structure and philosophy

[adammontville]

For example, we are focused on what we've been calling "level 1" measures, leaving "level 2" measures to a later revision. We also have issues regarding our "guiding principles", which would be useful to include in the front matter of this specification, to provide additional context to the users.

[adammontville]

• Update root index.rst text (i.e. replace "TBD - lots of descriptions" with a quick overview and proposed steps for "next reading")
• Add contribution information at about/contributing.rst and include in root index.rst after "Getting Started"
• ~Add license information at about/license.rst and include in root index.rst after "Contributing"~
• Update about/methodology.rst to include measure structure and philosophical considerations
  • L1 vs. L2 measures
  • Guiding principles
  • Measure structure