add preview img

Readme.rst

 Installation
 ===============
 
-Required packages in R: ``dplyr, ggplot2, reshape, knitr``. Use ``install.packages('pkngname')`` in R shell.
+Required packages in R: ``dplyr, ggplot2, reshape, knitr``. Use ``install.packages('pkngname')`` in the R shell.
 
 
 Usage
@@ -35,3 +35,6 @@ Execution
 - (optional) You can use emacs/`ess-mode <http://ess.r-project.org/>`_ to run the
   ``Rnw`` - file. This is more convenient, especially if you would like to make
   modifications. Here is how to set it up: http://yihui.name/knitr/demo/editors/
+
+
+.. figure:: preview.png