add img url ssh

external/contribute/mirror-fork/mirror-fork.md

@@ -16,6 +16,9 @@ you should follow the follow simple steps.
 1. Go to `Settings > Repository` (bottom left of the screen)
 2. Expand the section on `Mirroring repositories`
 3. In the field `Git repository URL`, enter the SSH clone address from the main repository. In case of How-to cards repository, the address is `ssh://git@gitlab.lcsb.uni.lu:8022/R3/howto-cards.git`
+
+ <img src="img/img1.png">
+
 4. Select `Mirror Direction` as `Pull`
 5. Click on `Detect Host Keys`
 6. `Authentication method` should be selected as `SSH public key`