Merge introductory text into a single document.

[Gulluth]

The SRD was originally a direct conversion from print to web. This created a few issues such as the web still referring to itself as a zine (#26), but also a haphazard merging of the introductory "welcome to vaarn" text.

With the removal of links to 3rd party stores (#27) the resulting landing page will become sparse, also I feel like the contents of README.md and what-is-vaarn.md could be merged into a single document that is served up as the landing page when folks visit the SRD. This will be similar to the editing that was done in the DE.