Remove README

None of this applies to the Volt version
Rename "jinja" to "volt"

This introduces no functional changes
Remove comment headers

These weren't even correct for the version I was copying from
Remove support for Vim versions < 6

Vim 5 is absolutely ancient and I won't be testing it
Base syntax upon PHP instead of HTML

Volt templates can contain PHP tags. Make Volt use PHP as a basis for
indent/syntax instead of HTML