Merge branch 'package-update' into 'master'

Tips for updating CE to EE with RPMs

I got an annoying error today while trying to upgrade CE to EE on Centos.

See merge request !259

doc/install/packages.md

@@ -8,6 +8,20 @@ The [GitLab Omnibus Readme](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/ma
 
 Please contact <subscribers@gitlab.com> if you have any questions.
 
+## RPM 'package is already installed' error
+
+If you are using RPM and you are upgrading from GitLab Community Edition you may get an error like this:
+
+```
+package gitlab-7.5.2_omnibus.5.2.1.ci-1.el7.x86_64 (which is newer than gitlab-7.5.2_ee.omnibus.5.2.1.ci-1.el7.x86_64) is already installed
+```
+
+You can override this version check with the `--oldpackage` option:
+
+```
+rpm -Uvh --oldpackage gitlab-7.5.2_ee.omnibus.5.2.1.ci-1.el7.x86_64.rpm
+```
+
 ## GitLab 7.5 Enterprise Edition
 
 - 7.5.3-ee/CI 5.2.1 - Ubuntu 14.04 64-bit [gitlab_7.5.3-ee.omnibus.5.2.1.ci-1_amd64.deb](https://s3-eu-west-1.amazonaws.com/downloads-packages/05998bb8ba9cca0ef22c90f4529c875003c51f23/ubuntu-14.04/gitlab_7.5.3-ee.omnibus.5.2.1.ci-1_amd64.deb)