Document how to write and implement Builder plugins

[Blackbaud-SteveBrush]

We need to document how to write and implement skyux-builder-plugin- plugins.

Here's an example plugin:
https://github.com/blackbaud/skyux-builder-plugin-stache

This pull request is related: blackbaud/skyux-builder#156

package.json

{
  "dependencies": {
    "@blackbaud/skyux-builder-plugin-stache": "^1.0.0"
  }
}

skyuxconfig.json

{
  "plugins": [
    "@blackbaud/skyux-builder-plugin-stache"
  ]
}

Plugin's index.js file:
The simplest form of a plugin would have an index.js file that includes:

module.exports = {
  // content = a string representation of the file's contents
  // resourcePath = the absolute path to the file
  // skyPagesConfig = the prepared config that includes skyuxconfig.json and builder's defaults
  preload(content, resourcePath, skyPagesConfig) {
    return content;
  };
}

[blackbaud-johnly]

Addressed in #456.