[Docs] Update some docs for the Open VSX Registry

Resolves #16.

This extension has been published to https://open-vsx.org/extension/rubocop/vscode-rubocop.
So, this commit updates some documentation to reflect the extension's distribution on the Open VSX Registry.

README.md

@@ -7,6 +7,8 @@ This is the official VS Code extension for [RuboCop](https://github.com/rubocop/
 
 You can install this VS Code extension from the [Visual Studio Marketplace](https://marketplace.visualstudio.com/items?itemName=rubocop.vscode-rubocop).
 
+For VS Code-based IDEs like VSCodium or Eclipse Theia, the extension can be installed from the [Open VSX Registry](https://open-vsx.org/extension/rubocop/vscode-rubocop).
+
 ## Language Server Capabilities
 
 These are the capabilities of this extension, each enabled by RuboCop's [built-in LSP server](https://docs.rubocop.org/rubocop/usage/lsp.html).

RELEASE.md

@@ -3,8 +3,11 @@ To cut a release of the extension.
 1. Update `version` in package.json and CHANGELOG.md
 2. Commit and make sure the CI is green
 3. Add a release tag with the git command and push to GitHub
+4. Release to [Visual Studio Marketplace](https://marketplace.visualstudio.com/items?itemName=rubocop.vscode-rubocop)
+and [Open VSX Registry](https://open-vsx.org/extension/rubocop/vscode-rubocop).
 
 You can published the extension to Visual Studio Marketplace with either A or B below.
+And, for releasing to the Open VSX Registry, please refer to resources like https://www.gitpod.io/blog/open-vsx#publishing.
 
 ## A. Release on web site of Visual Studio Marketplace