details on looking for correct installation guide

addresses gitlabhq/gitlabhq#3809

doc/install/installation.md

+# Select Version to Install
+Make sure you view this installation guide from the branch (version) of GitLab you would like to install. In most cases
+this should be the highest numbered stable branch (example shown below). 
+
+![capture](https://f.cloud.github.com/assets/1192780/564911/2f9f3e1e-c5b7-11e2-9f89-98e527d1adec.png)
+
+If this is unclear check the [GitLab Blog](http://blog.gitlab.org/) for installation guide links by version.
+
 # Important notes
 
 This installation guide was created for and tested on **Debian/Ubuntu** operating systems. Please read [`doc/install/requirements.md`](./requirements.md) for hardware and operating system requirements.