Add notes about localBuild and plugins

[joshuataylor]

Pull Request Details

Adds more details about how to add the plugin to a local plugin project, not sure if Gradle has changed since the original notes were written, but I needed to do this.

Description

The notes could be expanded to do something like this from S.O:

if (System.getProperty('USE_LOCAL_INTELLI_PLATFORM_GRADLE') == true) {
  includeBuild("/path/to/gradle-intellij-plugin")
}

Motivation and Context

Helping to fix and figure out issues!

How Has This Been Tested

I've ran this locally and can see the plugin now in my local plugin!

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have included my change in the CHANGELOG.
• I have added tests to cover my changes.
• All new and existing tests passed.