Add a short README section about the RubyGems version

Author: deivid-rodriguez

README.md

@@ -139,6 +139,14 @@ and the [condition and expression syntax](https://help.github.com/en/actions/ref
 The `working-directory` input can be set to resolve `.ruby-version`, `.tool-versions` and `Gemfile.lock`
 if they are not at the root of the repository, see [action.yml](action.yml) for details.
 
+## RubyGems
+
+By default, the default RubyGems version that comes with each Ruby is used.
+However, users can optionally customize the RubyGems version that they want by
+setting the `rubygems` input. See
+
+See [action.yml](action.yml) for more details about the `rubygems` input.
+
 ### Bundler
 
 By default, if there is a `Gemfile.lock` file (or `$BUNDLE_GEMFILE.lock` or `gems.locked`) with a `BUNDLED WITH` section,