git-merge: add --no-ff example

Include an example and brief description of the --no-ff option

pages/common/git-merge.md

@@ -9,3 +9,7 @@
 - Edit the merge message:
 
 `git merge -e {{branch_name}}`
+
+- Merge a branch and create a merge commit:
+
+`git merge --no-ff {{branch_name}}`