New pass through RFC-3; various improvements:

*   Add the "Package Name" type that disallows leading digits.
    [Discussion](#2 (comment)).
*   Document the Number type and allow `0` as a valid value for a
    Version Range.
*   Add the "Path Pattern" type and use it for the "ignore" property.
*   Merge "License" into "License Expression"
*   Rename "Version" to "SemVer", to distinguish it from any other
    versions used for dependencies.
*   Make "Version Range" not-specific to SemVers, since it's used for
    all sorts of dependency version requirements. Also, disallow version
    truncation in Version Ranges, since different version formats will
    have different rules.
*   Document that purl version expressions are valid but ignored.
*   Fix various spelling, grammatical, syntax, and narrative errors and
    clumsiness.
*   Update spec URLs to point to rfcs.pgxn.org.
*   Rename `generated_by` to `producer`.
*   Add question about preloading.
*   Note quality binary distribution as sign of success.