Update changelog

Author: JamesDoingStuff

changelog.md

@@ -7,6 +7,8 @@ SciReactUI Changelog
 ### Added
 - New Progress component based on Diamond Light added.
 - New ProgressDelayed component so that the progress isn't shown at all when it's a small wait.
+- NavMenu component added for creating dropdown menus in the NavBar
+  - NavMenuLink component extends NavLink to work in the NavMenu
 
 ### Fixed
 - Hovering over a slot caused a popup with the slot title in. This has been removed.