Add documentation for testing with TruffleRuby

Author: eregon

user/languages/ruby.md

@@ -22,6 +22,7 @@ language: ruby
 rvm:
   - 2.2
   - jruby
+  - truffleruby
   - 2.0.0-p247
 ```
 {: data-file=".travis.yml"}
@@ -46,6 +47,7 @@ language: ruby
 rvm:
   - 2.2
   - jruby
+  - truffleruby
   - 2.0.0-p247
 ```
 {: data-file=".travis.yml"}
@@ -79,6 +81,23 @@ rvm:
 ```
 {: data-file=".travis.yml"}
 
+### TruffleRuby
+
+To test with [TruffleRuby](https://github.com/oracle/truffleruby), simply add
+`truffleruby` or `truffleruby-VERSION` to your `.travis.yml`:
+```yaml
+language: ruby
+rvm:
+  - truffleruby # latest release
+  # or
+  - truffleruby-1.0.0-rc9 # specific version
+```
+{: data-file=".travis.yml"}
+
+See the [TruffleRuby releases](https://github.com/oracle/truffleruby/releases)
+page for a list of release versions.
+Please file any issue on [GitHub](https://github.com/oracle/truffleruby/issues).
+
 ### JRuby: C extensions are not supported
 
 Please note that **C extensions are not supported in JRuby** on Travis CI. The