Fix 9.1 Gitaly source update steps

doc/update/9.0-to-9.1.md

@@ -317,6 +317,17 @@ the socket path, but with `unix:` in front.
 
 Each entry under `storages:` should use the same `gitaly_address`.
 
+#### Compile Gitaly
+
+This step will also create `config.toml.example` which you need below.
+
+```shell
+cd /home/git/gitaly
+sudo -u git -H git fetch --all --tags
+sudo -u git -H git checkout v$(</home/git/gitlab/GITALY_SERVER_VERSION)
+sudo -u git -H make
+```
+
 #### Gitaly config.toml
 
 In GitLab 9.1 we are replacing environment variables in Gitaly with a