Added source upgrade guides for 12.0.0

doc/update/upgrading_from_source.md

@@ -4,6 +4,10 @@ comments: false
 
 # Upgrading Community Edition and Enterprise Edition from source
 
+NOTE: **Note:**
+Users wishing to upgrade to 12.0.0 will have to take some extra steps. See the
+version specific upgrade instructions for 12.0.0 for more details.
+
 Make sure you view this update guide from the branch (version) of GitLab you
 would like to install (e.g., `11.8`. You can select the version in the version
 dropdown at the top left corner of GitLab (below the menu bar).
@@ -404,6 +408,23 @@ Example:
 Additional instructions here.
 -->
 
+### 12.0.0
+
+In 12.0.0 we made various database related changes. These changes require that
+users first upgrade to the latest 11.11 patch release. Once upgraded to 11.11.x,
+users can upgrade to 12.0.x. You **can not** upgrade from 11.11.x to 12.1.0 or a
+newer version, instead you **must** first upgrade to 12.0.0. Failure to do so
+may result in database migrations not being applied, which could lead to
+application errors.
+
+Example 1: you are currently using GitLab 11.11.3, which is the latest patch
+release for 11.11.x. To upgrade, first upgrade to 12.0.0, then upgrade to any
+future versions.
+
+Example 2: you are currently using a version of GitLab 10.x. To upgrade, first
+upgrade to 11.11.3. Once upgraded to 11.11.3 you can safely upgrade to 12.0.0,
+then upgrade to any future versions.
+
 ## Things went south? Revert to previous version
 
 ### 1. Revert the code to the previous version