The updated contribution guidelines and branching documentation is
intended to add more predictability and clarity for potential
contributors. In some cases, students made pull requests targetting
master, which had to be rejected to avoid causing the master branch to
become unpredictable. Had I provided a better approach, it would've been
a better experience for them in general.

In order better handle this and similar situations, I've added a few
main branches to track the tree and documented what each one is used for
in a new BRANCHES.md document. Most of the documentation is based on
proven and scalable strategies straight from the Git repository itself.

Prior to this change, the branching strategy was more of an ad-hoc
process, which would sometimes cause issues to be merged into master,
defeating its purpose of being kept in a "production-ready" state at all
times.

With the new approach, there're four branches: `master`, `maint`, `next`,
and `pu` for current stable/production releases, maintenance, future
releases, and proposed updates respectively. These are explained in the
BRANCHES.md file in more detail.

The last release prior to this change is v0.2.0.

Signed-off-by: Raymond L. Rivera <ray.l.rivera@gmail.com>