Merge branch 'update_guide' into 'master'

Added update guide for 8.11

## What does this MR do?

I need to add notes about Elastic on EE side. To not create ugly conflicts in the future and to not duplicate work I created it here first.

I followed this guide https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#gitlab-ce

See merge request !5545

doc/update/8.10-to-8.11.md

+# From 8.10 to 8.11
+
+Make sure you view this update guide from the tag (version) of GitLab you would
+like to install. In most cases this should be the highest numbered production
+tag (without rc in it). You can select the tag in the version dropdown at the
+top left corner of GitLab (below the menu bar).
+
+If the highest number stable branch is unclear please check the
+[GitLab Blog](https://about.gitlab.com/blog/archives.html) for installation
+guide links by version.
+
+### 1. Stop server
+
+    sudo service gitlab stop
+
+### 2. Backup
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+```
+
+### 3. Get latest code
+
+```bash
+sudo -u git -H git fetch --all
+sudo -u git -H git checkout -- db/schema.rb # local changes will be restored automatically
+```
+
+For GitLab Community Edition:
+
+```bash
+sudo -u git -H git checkout 8-11-stable
+```
+
+OR
+
+For GitLab Enterprise Edition:
+
+```bash
+sudo -u git -H git checkout 8-11-stable-ee
+```
+
+### 4. Update gitlab-shell
+
+```bash
+cd /home/git/gitlab-shell
+sudo -u git -H git fetch --all --tags
+sudo -u git -H git checkout v3.2.1
+```
+
+### 5. Update gitlab-workhorse
+
+Install and compile gitlab-workhorse. This requires
+[Go 1.5](https://golang.org/dl) which should already be on your system from
+GitLab 8.1.
+
+```bash
+cd /home/git/gitlab-workhorse
+sudo -u git -H git fetch --all
+sudo -u git -H git checkout v0.7.8
+sudo -u git -H make
+```
+
+### 6. Update MySQL permissions
+
+If you are using MySQL you need to grant the GitLab user the necessary
+permissions on the database:
+
+```bash
+# Login to MySQL
+mysql -u root -p
+
+# Grant the GitLab user the REFERENCES permission on the database
+GRANT REFERENCES ON `gitlabhq_production`.* TO 'git'@'localhost';
+
+# Quit the database session
+mysql> \q
+```
+
+### 7. Install libs, migrations, etc.
+
+```bash
+cd /home/git/gitlab
+
+# MySQL installations (note: the line below states '--without postgres')
+sudo -u git -H bundle install --without postgres development test --deployment
+
+# PostgreSQL installations (note: the line below states '--without mysql')
+sudo -u git -H bundle install --without mysql development test --deployment
+
+# Optional: clean up old gems
+sudo -u git -H bundle clean
+
+# Run database migrations
+sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
+
+# Clean up assets and cache
+sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS_ENV=production
+
+```
+
+### 8. Update configuration files
+
+#### New configuration options for `gitlab.yml`
+
+There are new configuration options available for [`gitlab.yml`](config/gitlab.yml.example). View them with the command below and apply them manually to your current `gitlab.yml`:
+
+```sh
+git diff origin/8-10-stable:config/gitlab.yml.example origin/8-11-stable:config/gitlab.yml.example
+```
+
+#### Git configuration
+
+Disable `git gc --auto` because GitLab runs `git gc` for us already.
+
+```sh
+sudo -u git -H git config --global gc.auto 0
+```
+
+#### Nginx configuration
+
+Ensure you're still up-to-date with the latest NGINX configuration changes:
+
+```sh
+# For HTTPS configurations
+git diff origin/8-10-stable:lib/support/nginx/gitlab-ssl origin/8-11-stable:lib/support/nginx/gitlab-ssl
+
+# For HTTP configurations
+git diff origin/8-10-stable:lib/support/nginx/gitlab origin/8-11-stable:lib/support/nginx/gitlab
+```
+
+If you are using Apache instead of NGINX please see the updated [Apache templates].
+Also note that because Apache does not support upstreams behind Unix sockets you
+will need to let gitlab-workhorse listen on a TCP port. You can do this
+via [/etc/default/gitlab].
+
+[Apache templates]: https://gitlab.com/gitlab-org/gitlab-recipes/tree/master/web-server/apache
+[/etc/default/gitlab]: https://gitlab.com/gitlab-org/gitlab-ce/blob/8-11-stable/lib/support/init.d/gitlab.default.example#L38
+
+#### SMTP configuration
+
+If you're installing from source and use SMTP to deliver mail, you will need to add the following line
+to config/initializers/smtp_settings.rb:
+
+```ruby
+ActionMailer::Base.delivery_method = :smtp
+```
+
+See [smtp_settings.rb.sample] as an example.
+
+[smtp_settings.rb.sample]:  https://gitlab.com/gitlab-org/gitlab-ce/blob/8-11-stable/config/initializers/smtp_settings.rb.sample#L13?
+
+#### Init script
+
+Ensure you're still up-to-date with the latest init script changes:
+
+    sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
+
+### 9. Start application
+
+    sudo service gitlab start
+    sudo service nginx restart
+
+### 10. Check application status
+
+Check if GitLab and its environment are configured correctly:
+
+    sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production
+
+To make sure you didn't miss anything run a more thorough check:
+
+    sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
+
+If all items are green, then congratulations, the upgrade is complete!
+
+## Things went south? Revert to previous version (8.10)
+
+### 1. Revert the code to the previous version
+
+Follow the [upgrade guide from 8.9 to 8.10](8.9-to-8.10.md), except for the
+database migration (the backup is already migrated to the previous version).
+
+### 2. Restore from the backup
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
+```
+
+If you have more than one backup `*.tar` file(s) please add `BACKUP=timestamp_of_backup` to the command above.