Merge pull request #3126 from dosire/refer-to-vagrant-vm

Refer to virtual machine from the installation document.

doc/install/installation.md

-This installation guide was created for Debian/Ubuntu and tested on it.
-
-Please read [`doc/install/requirements.md`](./requirements.md) for hardware and platform requirements.
+This installation guide was created for Debian/Ubuntu and tested on it. Please read [`doc/install/requirements.md`](./requirements.md) for hardware and platform requirements.
 
+This installation guide is recommended to set up a production server. If you want a development environment please use the [Vargrant virtual machine](https://github.com/gitlabhq/gitlab-vagrant-vm) since it makes it much easier to set up all the dependencies for integration testing.
 
 **Important Note:**
 The following steps have been known to work.