Merge pull request #4130 from TrevorS/fix-update-docs-5.0-5.1

adding update info regarding gitlab-shell config.yml (re: dashboard update problems)

doc/update/5.0-to-5.1.md

@@ -23,8 +23,14 @@ sudo -u git -H git checkout 5-1-stable
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch
 sudo -u git -H git checkout v1.3.0
+# replace your old config with the new one
+sudo -u git -H mv config.yml config.yml.old
+sudo -u git -H cp config.yml.example config.yml
+# edit options to match old config
+sudo -u git -H vi config.yml
 ```
 
+
 ### 4. Install libs, migrations etc
 
 ```bash