Merge pull request #4913 from radeksimko/x11-doc

Mention x11_required caveat in docs

doc/CASK_LANGUAGE_REFERENCE.md

@@ -149,6 +149,7 @@ The following methods may be called to generate standard warning messages:
 | `files_in_usr_local`              | The Cask installs files to `/usr/local`, which may confuse Homebrew
 | `arch_only(list)`                 | The Cask only supports certain architectures.  Currently valid elements of `list` are `intel-32` and `intel-64`
 | `os_version_only(list)`           | The Cask only supports certain OS X Versions.  Currently valid elements of `list` are `10.5`, `10.6`, `10.7`, `10.8`, and `10.9`
+| `x11_required`                    | The Cask requires X11 to run
 
 Example: