Update docs for 5.2

doc/update/5.1-to-5.2.md

+# From 5.1 to 5.2
+
+### 1. Stop server
+
+    sudo service gitlab stop
+
+### 2. Get latest code
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git fetch
+sudo -u git -H git checkout 5-2-stable
+```
+
+### 3. Update gitlab-shell
+
+```bash
+cd /home/git/gitlab-shell
+sudo -u git -H git fetch
+sudo -u git -H git checkout v1.4.0
+```
+
+### 4. Install libs, migrations etc
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle install --without development test postgres --deployment
+sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
+```
+
+### 5. Start application
+
+    sudo service gitlab start