Add relative URL documentation

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diff --git a/doc/install/installation.md b/doc/install/installation.md
index 3eb9b1767c5..86d37d64edd 100644
--- a/doc/install/installation.md
+++ b/doc/install/installation.md
@@ -484,6 +484,11 @@ You can use `sudo service gitlab start` and `sudo service gitlab stop` to start
 
 ## Advanced Setup Tips
 
+### Relative URL support
+
+See the [Relative URL documentation](relative_url.md) for more information on
+how to set this up.
+
 ### Using HTTPS
 
 To use GitLab with HTTPS:
diff --git a/doc/install/relative_url.md b/doc/install/relative_url.md
new file mode 100644
index 00000000000..e6d530e2729
--- /dev/null
+++ b/doc/install/relative_url.md
@@ -0,0 +1,123 @@
+## Install GitLab under a relative URL
+
+_**Note:**
+This document describes how to run GitLab under a relative URL for installations
+from source. If you are using an Omnibus package,
+[the steps are different][omnibus-rel]. Use this guide along with the
+[installation guide](installation.md) if you are installing GitLab for the
+first time._
+
+---
+
+While it is recommended to install GitLab in its own (sub)domain, sometimes
+this is not possible due to a variety of reasons. In that case, GitLab can also
+be installed under a relative URL, for example `https://example.com/gitlab`.
+
+There is no limit as to how much deeply nested the relative URL can be. For
+example you could serve GitLab under `/foo/bar/gitlab/git` without any issues.
+
+Note that by changing the URL on an existing GitLab installation, all remote
+URLS will change, so you'll have to manually edit them in any local repository
+that points to your GitLab instance.
+
+---
+
+The TL;DR list of configuration files that you need to change in order to
+serve GitLab under a relative URL is:
+
+- `/home/git/gitlab/config/application.rb`
+- `/home/git/gitlab/config/gitlab.yml`
+- `/home/git/gitlab/config/unicorn.rb`
+- `/home/git/gitlab-shell/config.yml`
+- `/etc/default/gitlab`
+
+After all the changes you need to recompile the assets and [restart GitLab].
+
+### Relative URL requirements
+
+If you configure GitLab with a relative URL, the assets (JavaScript, CSS, fonts,
+images, etc.) will need to be recompiled, which is a task which consumes a lot
+of CPU and memory resources. To avoid out-of-memory errors, you should have at
+least 2GB of RAM available on your system, while we recommend 4GB RAM, and 4 or
+8 CPU cores.
+
+See the [requirements](requirements.md) document for more information.
+
+### Enable relative URL in GitLab
+
+_**Note:**
+Do not make any changes to your web server configuration file regarding
+relative URL. The relative URL support is basically implemented by GitLab
+Workhorse._
+
+---
+
+Before following the steps below to enable relative URL in GitLab, some
+assumptions are made:
+
+- GitLab is served under `/gitlab`
+- The directory under which GitLab is installed is `/home/git/`
+
+Make sure to follow all steps below:
+
+1.  (Optional) If you run short on resources, you can temporarily free up some
+    memory by shutting down the GitLab service with the following command:
+
+    ```shell
+    sudo service gitlab stop
+    ```
+
+1.  Edit `/home/git/gitlab/config/application.rb` and uncomment/change the
+    following line:
+
+    ```ruby
+    config.relative_url_root = "/gitlab"
+    ```
+
+1.  Edit `/home/git/gitlab/config/gitlab.yml` and uncomment/change the
+    following line:
+
+    ```yaml
+    relative_url_root: /gitlab
+    ```
+
+1.  Edit `/home/git/gitlab/config/unicorn.rb` and uncomment/change the
+    following line:
+
+    ```ruby
+    ENV['RAILS_RELATIVE_URL_ROOT'] = "/gitlab"
+    ```
+
+1.  Edit `/home/git/gitlab-shell/config.yml` and append the relative path to
+    the following line:
+
+    ```yaml
+    gitlab_url: http://127.0.0.1/gitlab
+    ```
+
+1.  Make sure you have copied the supplied init script and the defaults file
+    as stated in the [installation guide](installation.md#install-init-script).
+    Then, edit `/etc/default/gitlab` and set in `gitlab_workhorse_options` the
+    `-authBackend` to read like:
+
+    ```shell
+    -authBackend http://127.0.0.1:8080/gitlab
+    ```
+
+1.  After all the above changes recompile the assets. This is an important task
+    and will take some time to complete depending on the server resources.
+
+    ```
+    cd /home/git/gitlab
+    sudo -u git -H bundle exec rake assets:clean assets:precompile RAILS_ENV=production
+    ```
+
+1. [Restart GitLab][] for the changes to take effect.
+
+### Disable relative URL in GitLab
+
+To disable the relative URL, follow the same steps as above and set up the
+GitLab URL to a one that doesn't contain a relative path.
+
+[omnibus-rel]: http://doc.gitlab.com/omnibus/settings/configuration.html#configuring-a-relative-url-for-gitlab "How to setup relative URL in Omnibus GitLab"
+[restart gitlab]: ../administration/restart_gitlab.md#installations-from-source "How to restart GitLab"