added .rsx extension to R. 

When we write R extensions for QGIS we have to end the files in .rsx [1]. But the code its pure R. It would be great if github acknowledge those scripts as R scripts.

[1] http://www.qgis.org/es/docs/user_manual/processing/3rdParty.html#r-creating-r-scripts
Added sample of R qgis script
Merge pull request #944 from midraed/patch-1

added .rsx extension to R languge