Added documentation on how to downgrade from EE to CE

doc/README.md

@@ -55,6 +55,7 @@
 - [Migrate GitLab CI to CE/EE](migrate_ci_to_ce/README.md) Follow this guide to migrate your existing GitLab CI data to GitLab CE/EE.
 - [User permissions](permissions/permissions.md)
 - [API](api/README.md)
+- [Downgrade back to CE](downgrade_ee_to_ce/README.md) Follow this guide if you need to downgrade from EE to CE.
 
 ## Contributor documentation
 

doc/downgrade_ee_to_ce/README.md

+# Downgrading from EE to CE
+
+If you ever decide to downgrade your Enterprise Edition back to the Community Edition, there are a few
+steps you need take before installing the CE package on top of the current EE package, or, if you are
+in an installation from source, before you change remotes and fetch the latest CE code.
+
+## Disable Enterprise only features
+
+First thing to do is to disable the following features.
+
+### Authentication mechanisms
+
+Kerberos and Atlassian Crowd are only available on the Enterprise Edition, so you should disable these mechanisms before
+downgrading and you should provide alternative authentication methods to your users.
+
+### Git Annex
+
+Git Annex is also only available on the Enterprise Edition. This means that if you have repositories that use Git Annex
+to store large files, these files will no longer be easily available via Git. You should consider migrating these repositories
+to use Git LFS before downgrading to the Community Edition.
+
+### Remove Jenkins CI Service entries from the database
+
+The `JenkinsService` class is only available on the Enterprise Edition codebase, so if you downgrade to
+the Community Edition, you'll come across the following error:
+
+```
+Completed 500 Internal Server Error in 497ms (ActiveRecord: 32.2ms)
+
+ActionView::Template::Error (The single-table inheritance mechanism failed to locate the subclass: 'JenkinsService'. This
+error is raised because the column 'type' is reserved for storing the class in case of inheritance. Please rename this
+column if you didn't intend it to be used for storing the inheritance class or overwrite Service.inheritance_column to
+use another column for that information.)
+```
+
+All services are created automatically for every project you have, so in order to avoid getting this error, you need to
+remove all instances of the `JenkinsService` from your database:
+
+**Omnibus Installation**
+
+```
+$ sudo gitlab-rails runner "Service.where(type: 'JenkinsService').delete_all"
+```
+
+**Source Installation**
+
+```
+$ bundle exec rails runner "Service.where(type: 'JenkinsService').delete_all" production
+```
+
+## Downgrade to CE
+
+After performing the above mentioned steps, you are now ready to downgrade your GitLab installation to the Community
+Edition.
+
+**Omnibus Installation**
+
+To downgrade an Omnibus installation, it is sufficient to install the Community Edition package on top of the currently
+installed one. You can do this manually, by directly [downloading the package](https://packages.gitlab.com/gitlab/gitlab-ce)
+you need, or by adding our CE package repository and following the [CE installation instructions.](https://about.gitlab.com/downloads/)
+
+**Source Installation**
+
+To downgrade a source installation, you need to replace the current remote of your GitLab installation with the Community
+Edition's remote, fetch the latest changes, and checkout the latest stable branch:
+
+```
+$ git remote set-url origin git@gitlab.com:gitlab-org/gitlab-ce.git
+$ git fetch --all
+$ git checkout 8-x-stable
+```
+
+Remember to follow the correct [update guides](../update/README.md) to make sure all dependencies are up to date.
\ No newline at end of file

doc/update/README.md

@@ -4,6 +4,11 @@ Depending on the installation method and your GitLab version, there are multiple
 
 - [The CE to EE update guides](https://gitlab.com/gitlab-org/gitlab-ee/tree/master/doc/update) the steps are very similar to a version upgrade: stop the server, get the code, update config files for the new functionality, install libs and do migrations, update the init script, start the application and check the application status.
 
+## EE to CE
+
+- If you need to downgrade your EE installation back to CE, you can follow [this guide](../downgrade_ee_to_ce/README.md) to make
+the process as smooth as possible.
+
 ## Omnibus Packages
 
 - [Omnibus update guide](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/update.md) contains the steps needed to update a GitLab [package](https://about.gitlab.com/downloads/).