add preliminary 7.6 upgrade guide

Add preliminary 7.6 upgrade guide. Makes it easier to add upgrades as changes are made rather than trying to round up everything at RC1.

Initial additions:
* Nginx changes needed again in 7.6 as they did not make the final 7.5 upgrade guide
* Suggest that user sets time zone (added in https://github.com/gitlabhq/gitlabhq/pull/8015 but missed in final 7.5 upgrade guide)

Replaces https://github.com/gitlabhq/gitlabhq/pull/8124

doc/update/7.5-to-7.6.md

+# From 7.5 to 7.6
+
+**7.6 is not yet released. This is a preliminary upgrade guide.**
+
+### 0. Stop server
+
+    sudo service gitlab stop
+
+### 1. Backup
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+```
+
+### 2. 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 7-6-stable
+```
+
+OR
+
+For GitLab Enterprise Edition:
+
+```bash
+sudo -u git -H git checkout 7-6-stable-ee
+```
+
+### 3. Update gitlab-shell
+
+```bash
+cd /home/git/gitlab-shell
+sudo -u git -H git fetch
+sudo -u git -H git checkout v2.2.0
+```
+
+### 4. 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 development test postgres --deployment
+
+# PostgreSQL installations (note: the line below states '--without ... mysql')
+sudo -u git -H bundle install --without development test mysql --deployment
+
+# 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
+
+# Update init.d script
+sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
+```
+
+### 5. Update config 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 to your current `gitlab.yml`.
+
+```
+git diff origin/7-5-stable:config/gitlab.yml.example origin/7-6-stable:config/gitlab.yml.example
+```
+
+#### Change Nginx settings
+
+* HTTP setups: Make `/etc/nginx/sites-available/gitlab` the same as [`lib/support/nginx/gitlab`](/lib/support/nginx/gitlab) but with your settings
+* HTTPS setups: Make `/etc/nginx/sites-available/gitlab-ssl` the same as [`lib/support/nginx/gitlab-ssl`](/lib/support/nginx/gitlab-ssl) but with your setting
+
+#### Setup time zone (optional)
+
+Consider setting the time zone in `gitlab.yml` otherwise GitLab will default to UTC. If you set a time zone previously in [`application.rb`](config/application.rb) (unlikely), unset it.
+
+### 6. Start application
+
+    sudo service gitlab start
+    sudo service nginx restart
+
+### 7. 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 with:
+
+    sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
+
+If all items are green, then congratulations upgrade is complete!
+
+## Things went south? Revert to previous version (7.5)
+
+### 1. Revert the code to the previous version
+Follow the [upgrade guide from 7.4 to 7.5](7.4-to-7.5.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.