DOC: Add License badge to README file

Add License badge to `README` file.

Author: jhlegarreta

README.md

@@ -1,5 +1,7 @@
 # SlicerWMA
 
+[![License](https://img.shields.io/badge/License-BSD-green.svg)](https://github.com/SlicerDMRI/SlicerWMA/blob/master/License.txt)
+
 This is a Slicer extension to provide access to Lauren O'Donnell's [whitematteranalysis](https://github.com/SlicerDMRI/whitematteranalysis) package.
 
 Currently the SlicerWMA extension provides a meta-package to install whitematteranalysis and all dependencies within the Slicer Python environment, in order to facilitate command line usage, as well as testing and graphical interface module development on multiple platforms.