Add an example for testing against all stable versions

* Test it in CI too.

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ 'ubuntu-16.04', 'ubuntu-18.04', 'macos-latest' ]
-        ruby: [ 'ruby-2.6.5', '2.7.0', 'truffleruby', 'jruby-9.2.9.0' ]
+        ruby: [ '2.4.9', '2.5.7', '2.6.5', '2.7.0', 'truffleruby', 'jruby' ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2

README.md

@@ -14,7 +14,7 @@ The prebuilt rubies are generated by https://github.com/eregon/ruby-install-buil
 
 ## Usage
 
-Single job:
+### Single Job
 
 ```yaml
 name: My workflow
@@ -30,7 +30,7 @@ jobs:
     - run: ruby -v
 ```
 
-Matrix:
+### Matrix
 
 ```yaml
 name: My workflow
@@ -52,9 +52,33 @@ jobs:
 ```
 
 If a specific version is not given, it uses the latest stable release of that implementation.
-
 For instance `truffleruby` is currently the same as `truffleruby-19.3.0`.
 
+If it's just a version number (e.g., `2.6.5`), then MRI is assumed (same as `ruby-2.6.5`).
+
+### All Stable Versions
+
+With that, we can test on all stable releases of MRI, JRuby and TruffleRuby with:
+
+```yaml
+name: My workflow
+on: [push]
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ 'ubuntu-latest', 'macos-latest' ]
+        ruby: [ '2.4.9', '2.5.7', '2.6.5', '2.7.0', 'truffleruby', 'jruby' ]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v2
+    - uses: eregon/use-ruby-action@master
+      with:
+        ruby-version: ${{ matrix.ruby }}
+    - run: ruby -v
+```
+
 ## Efficiency
 
 It takes about 5 seconds to setup the given Ruby.