Recommend the short version syntax

* It's safer long-term and ensures to use the latest release with the most bug fixes.

README.md

@@ -15,6 +15,10 @@ This action currently supports these versions of MRI, JRuby and TruffleRuby:
 | JRuby | 9.2.9.0 |
 | TruffleRuby  | 19.3.0 |
 
+The recommended way to refer to these versions is:
+`2.3`, `2.4`, `2.5`, `2.6`, `2.7`, `jruby`, `truffleruby`.  
+That way, the latest compatible release is used, which contains the most bug fixes.
+
 See https://github.com/eregon/ruby-install-builder/blob/metadata/versions.json
 for the always up-to-date list of available Ruby versions.
 

action.yml

@@ -6,7 +6,7 @@ branding:
   icon: download
 inputs:
   ruby-version:
-    description: 'Engine and version to use in the format "myruby-X.Y.Z"'
+    description: 'Engine and version to use, see the syntax in the README'
     required: true
 outputs:
   ruby-prefix: