Update contributing.md

docs/src/contributing.md

@@ -1,8 +1,10 @@
 # Contributing
 
 Trixi.jl is an open-source project and we are very happy to accept contributions
-from the community. Please feel free to open issues or submit patches (preferably
-as merge requests) any time. For planned larger contributions, it is often
+from the community. Please feel free to 
+[open issues](https://github.com/trixi-framework/Trixi.jl/issues/new/choose)
+or submit patches (preferably as [pull requests](https://github.com/trixi-framework/Trixi.jl/pulls)) 
+any time. For planned larger contributions, it is often
 beneficial to get in contact with one of the principal developers first (see
 [Authors](@ref)).