Update installation instructions

README.md

@@ -4,7 +4,23 @@ Numix is a part of the [Numix Project](http://numixproject.org).
 
 ### Manual installation
 
-Extract the zip file to the themes directory i.e. `/usr/share/themes/`
+First, you need to compile the theme using the [Sass](http://sass-lang.com/) compiler.
+
+To install Sass, install ruby and the gem command using your distro's package manager. Then install `sass` with the `gem` command,
+
+`gem install sass`
+
+You'll also need the following commands in your path to generate the gresource binary. Install them using your distro's package manager.
+
+* `glib-compile-schemas`
+* `gdk-pixbuf-pixdata`
+
+After installing all the dependencies, switch to the cloned directory and, run the following in Terminal,
+
+```
+make
+sudo make install
+```
 
 To set the theme in Gnome, run the following commands in Terminal,