Add signature help to the list!

Author: Winston Liu

README.md

@@ -18,6 +18,7 @@ This package is currently an early access release.  You should also install the
 * Go to definition
 * Hover
 * Reference highlighting
+* Signature help
 
 ## Contributing
 Always feel free to help out!  Whether it's [filing bugs and feature requests](https://github.com/atom/languageserver-java/issues/new) or working on some of the [open issues](https://github.com/atom/languageserver-java/issues), Atom's [contributing guide](https://github.com/atom/atom/blob/master/CONTRIBUTING.md) will help get you started while the [guide for contributing to packages](https://github.com/atom/atom/blob/master/docs/contributing-to-packages.md) has some extra information.