A New Branching Strategy for xPDO

In an attempt to simplify the contribution process for xPDO and bring it inline with the new MODX Revolution strategy, I am introducing a similar new branching strategy based on major and minor versions.

Major and Minor Version-Specific Branches

Instead of a permanent master and develop branch that continually change meaning as the stable releases are tagged, this simplified strategy maintains a development branch for every major release, e.g. 2.x, 3.x, etc., and a development branch for the current stable minor releases within each major version, e.g. 2.4.x. The major-version development branch represents changes targeted for the next minor release, while the minor-version development branch contains changes for the next patch release. Following SemVer, this means features always go to a major-version branch and bug fixes should go to a minor-version branch.