Update hardware and platform requirements guide

doc/install/installation.md

-_This installation guide created for Debian/Ubuntu and properly tested._
+This installation guide was created for Debian/Ubuntu and tested on it.
+
+Please read doc/install/requirements.md for hardware andplatform requirements.
 
-_Checkout requirements before setup_
 
 
 ### IMPORTANT

doc/install/requirements.md

-## Platform requirements:
+# Hardware
 
-**The project is designed for the Linux operating system.**
+We recommend you to run GitLab on a server with at least 1GB RAM.
 
-It may work on FreeBSD and Mac OS, but we don't test our application for these systems and can't guarantee stability and full functionality.
+The necessary hard disk space largely depends on the size of the repos you want
+to use GitLab with. But as a *rule of thumb* you should have at least as much
+free space as your all repos combined take up.
 
-We officially support (recent versions of) these Linux distributions:
+
+
+# Operating Systems
+
+## Linux
+
+GitLab is developed for the Linux operating system.
+
+GitLab officially supports (recent versions of) these Linux distributions:
 
 - Ubuntu Linux
 - Debian/GNU Linux
 
-It should work on:
+It should also work on (though they are not officially supported):
 
+- Arch
+- CentOS
 - Fedora
-- CentOs
+- Gentoo
 - RedHat
 
-You might have some luck using these, but no guarantees:
+## Other Unix Systems
+
+There is nothing that prevents GitLab from running on other Unix operating
+systems. This means you may get it to work on systems running FreeBSD or OS X.
+**If you want to try, please proceed with caution!**
+
+## Windows
+
+GitLab does **not** run on Windows and we have no plans of supporting it in the
+near future.
+
+
+
+# Rubies
 
-- FreeBSD will likely work, see https://github.com/gitlabhq/gitlabhq/issues/796
-- MacOS X will likely work, see https://groups.google.com/forum/#!topic/gitlabhq/5IXHbPkjKLA
+GitLab requires Ruby (MRI) 1.9.3 and several Gems with native components.
+While it is generally possible to use other Rubies (like
+[JRuby](http://jruby.org/) or [Rubinius](http://rubini.us/)) it might require
+some work on your part.
 
-GitLab does **not** run on Windows and we have no plans of making GitLab compatible.
 
 
-## Hardware: 
+# Installation troubles and reporting success or failure
 
-We recommend to use server with at least 1GB RAM for gitlab instance.
+If you have troubles installing GitLab following the official installation guide
+or want to share your experience installing GitLab on a not officially supported
+platform, please follow the the contribution guide (see CONTRIBUTING.md).