Proposal: Closure Templates Change and Release Policy

# Motivation

Closure Templates is a mature product with widespread use both inside Google and in the open-source world. In general, we don’t want to change the workflow for writing Closure Templates or the language features available to template authors. Rather, we want to **make it easier to use the best possible Closure Templates features** (and conversely, make it harder or impossible to use Closure Templates in an insecure or non-performant way). We think it is worthwhile to occasionally make breaking changes to Closure Templates in pursuit of this goal.
# Proposed Policy
1. Closure Templates will be released monthly, as close to the start of the month as possible. A release will consist of artifacts published to Maven Central, as well as a release tag in the GitHub repo.
2. We will keep a `BREAKING_CHANGES.md` file in the GitHub repo. This file will be updated every release to describe breaking changes since the last release (if any), and what steps open-source users need to take to upgrade.
3. For every release, we will make an announcement on closure-templates-discuss, pointing to the Maven artifacts, the GitHub tag, the breaking changes, etc.