Add a README.md for included plugins

Integrating the plugin marketplaces into VS Code, folks [noticed](https://github.com/microsoft/vscode/issues/297245) that "Open Readme" links for the skills in this repo 404'd. It'd be helpful to add readmes to the skills so we can show something useful here 🙂

Start by listing the included skills and plugins in the repository and checking their existing directory structure for README files. Add a README.md for each relevant skill so the VS Code “Open Readme” links resolve instead of returning 404s. Verify that every included skill intended for the marketplace has a usable README.

Written by the indexing model from the issue text.