[BUG] Add repo description

[elfisher]

Coming from opensearch-project/opensearch-plugins#92.

Each plugin has a short descriptive text blurb that says what it does: this appears in plugin-descriptor.properties (OpenSearch plugins) or package.json (OpenSearch Dashboards plugins), as well as on the project website's "source" page and in the "About" section of every repo. These were created one-by-one over the years as plugins were created, so looking at them all together now it would be hard for somebody new to the project to use these to understand what the plugins do.

[elfisher]

E.g., "Provides templates and resources for other OpenSearch project repositories"