Merge branch '5-3-update-guide' into '5-3-stable'

5.3 update guide

Added update guide from 5.2 to 5.3

See merge request !97

doc/update/5.2-to-5.3.md

+# Update from 5.2 to 5.3
+
+__GitLab CI 5.3 requires GitLab 7.5 or higher and GitLab CI Runner v5__
+
+### 1. stop CI server
+
+    sudo service gitlab_ci stop
+
+### 2. Switch to your gitlab_ci user
+
+```
+sudo su gitlab_ci
+cd /home/gitlab_ci/gitlab-ci
+```
+
+### 3. Get latest code
+
+```
+git fetch
+git checkout 5-3-stable
+```
+
+#### Redis config
+
+If you have `config/resque.yml` file - please update it with recent `config/resque.yml.example`
+
+### 4. Install libs, migrations etc
+
+```
+# For MySQL users
+bundle install --without postgres development test --deployment
+
+# For Postgres users
+bundle install --without mysql development test --deployment
+
+# Run migrations
+bundle exec rake db:migrate RAILS_ENV=production
+```
+
+### 5. Start web application
+
+    sudo service gitlab_ci start
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