Version numbering strategy

[johnbillion]

Proposed version numbering strategy for this library, taking into consideration that I may want to change the schema structure outside any given release of WordPress (eg. in #40).

x.y.z where:

• x is the schema major version number, currently 2
• y is the WordPress branch without its decimal place, currently 58, soon to be 59
• z is the schema patch version, currently 0, although I'll use this for minors too which is currently 12

This allows for sane, semver-compatible version constraints while still allowing a WordPress branch version to be specified:

• ~2.58 for schema version 2 for WordPress 5.8
• ^3.0 for schema version 3 for the latest WordPress version
• 4.61.0 for schema version 4 for WordPress 6.1 without allowing patch releases to the schema (not recommended)