Give a hint if users want exact or more versions

* Many Ruby users will want something like that as the issues on this
  repository make very clear.
* Once actions#44 is addressed
  we can remove this.

Author: eregon

README.md

@@ -14,6 +14,9 @@ Note that a `ruby-version:` of `2.6` or `2.6.x` are equivalent.
 
 > Supports `2.4`, `2.5`, `2.6`, and `2.7`.
 
+If you want to use exact versions (e.g., `2.6.5`) of Ruby, or to use JRuby or TruffleRuby,
+please take a look at other actions such as [use-ruby-action](https://github.com/eregon/use-ruby-action).
+
 # Usage
 
 See [action.yml](action.yml)