Add security warnings to update docs

doc/update/4.2-to-5.0.md

 # From 4.2 to 5.0
 
+## Warning
+GitLab 5.0 is affected by critical security vulnerability CVE-2013-4490. Please update to GitLab 5.4 immediately.
+
 ## Important changes
 
 * We don't use `gitlab` user any more. Everything will be moved to `git` user

doc/update/5.0-to-5.1.md

 # From 5.0 to 5.1
 
+## Warning
+GitLab 5.1 is affected by critical security vulnerability CVE-2013-4490. Please update to GitLab 5.4 immediately.
+
 ## Release notes:
 
 * `unicorn` replaced with `puma`

doc/update/5.1-to-5.2.md

 # From 5.1 to 5.2
 
+## Warning
+GitLab 5.2 is affected by critical security vulnerabilities CVE-2013-4490 and CVE-2013-4489. Please update to GitLab 5.4 immediately.
+
 ### 0. Backup
 
 It's useful to make a backup just in case things go south:

doc/update/5.1-to-6.0.md

 # From 5.1 to 6.0
 
+## Warning
+GitLab 6.0 is affected by critical security vulnerabilities CVE-2013-4490 and CVE-2013-4489. Please update to GitLab 6.2 or newer immediately.
+
 ### Deprecations
 
 #### Global projects

doc/update/5.2-to-5.3.md

 # From 5.2 to 5.3
 
+## Warning
+GitLab 5.3 is affected by critical security vulnerabilities CVE-2013-4490 and CVE-2013-4489. Please update to GitLab 5.4 immediately.
+
 ### 0. Backup
 
 It's useful to make a backup just in case things go south:

doc/update/5.3-to-5.4.md

 # From 5.3 to 5.4
 
+## Notice
+Security vulnerabilities CVE-2013-4490 and CVE-2013-4489 have been patched in the latest version of GitLab 5.4.
+
 ### 0. Backup
 
 It's useful to make a backup just in case things go south:

doc/update/5.4-to-6.0.md

 # From 5.4 to 6.0
 
+## Warning
+GitLab 6.0 is affected by critical security vulnerabilities CVE-2013-4490 and CVE-2013-4489. Please update to GitLab 6.2 or newer immediately.
+
 ### Deprecations
 
 #### Global projects

doc/update/6.0-to-6.1.md

 # From 6.0 to 6.1
 
+## Warning
+GitLab 6.1 is affected by critical security vulnerabilities CVE-2013-4490 and CVE-2013-4489. Please [update straight to 6.2](6.0-to-6.2.md).
+
 # In 6.1 we remove a lot of deprecated code.
 # You should update to 6.0 before installing 6.1 so all the necessary conversions are run.
 

doc/update/6.1-to-6.2.md

 # From 6.1 to 6.2
 
+## Notice
+Security vulnerabilities CVE-2013-4490 and CVE-2013-4489 have been patched in the latest version of GitLab 6.2.
+
 # You should update to 6.1 before installing 6.2 so all the necessary conversions are run.
 
 ### 0. Backup