Make OMCompiler-3rdParty contributions more clear (#8091)

CONTRIBUTING.md

@@ -48,5 +48,6 @@ If you need to make changes to OMCompiler-3rdParty the procedure is as follows:
 * make a PR to OpenModelica glue project with OpenModelica/OMCompiler/3rdParty submodule pointing at your commit from the pushed branch in OMCompiler-3rdParty
 
 After Jenkins checks that all is OK a developer will:
-* rebase the commit from the OMCompiler-3rdParty branch to master
+* **reset** (or restart, or **merge**, if there were other commits added to OMCompiler-3rdParty since you started) the OMCompiler-3rdParty master branch so the new HEAD contains the HEAD commit of the branch
 * merge the PR in the OpenModelica glue project
+* delete the branch in the OMCompiler-3rdParty