Introduce versioning/release strategy

@bmuschko commented on [Wed Aug 30 2017](https://github.com/gradle-guides/gradle-guides-plugin/issues/14)

The plugin was designed to publish a new version of the plugin with every commit. CI compiles the code, runs the tests, creates the archive and publishes it to the plugin portal. Publishing to the portal requires setting a new version for the project. Otherwise, publishing will fail and therefore the CI job.

The code is currently design to expect a change to the version with every commit. That's not desired. Sometimes you just want to update the docs or the build script but you don't necessarily want to publish a new version of the archive. The purpose of this issue to decouple these concerns.

Start by reviewing the CI workflow, project version configuration, and plugin-portal publishing process described in the issue. Determine how version changes are currently tied to every commit and define a release strategy that separates documentation or build-script updates from archive publication. Done means CI can handle non-release changes without requiring a new plugin version.

Written by the indexing model from the issue text.