Update vegetationsystem.adoc

Added link to contributions repository.

src/docs/asciidoc/jme3/contributions/vegetationsystem.adoc

@@ -11,6 +11,7 @@ This is the wiki page for the Forester grass/tree loading system. It contains in
 
 Topics are flagged as (basic) or (advanced). Advanced topics can be skipped.
 
+The library can be found in the Github link:https://github.com/jMonkeyEngine-Contributions[jMonkeyEngine Contributions repository].
 
 == The Forester (Basic)