Change rules to simplify PR analysis

CONTRIBUTING.md

@@ -13,6 +13,7 @@ This is a compilation of **Free and Libre Open Source Software ([FLOSS](https://
   
 ## Pull Requests
 * Use imperative (e.g. add instead of added/adding) in the PR title.
+* Insert only one software per PR.
 * Include application names, categories and a link to the open source project page in the description.
 
 ## Issues