Added Zenodo DOI badge to pages

Author: stiber

docs/index.html

@@ -20,7 +20,7 @@
         <header>
           <h1>Signal-computing</h1>
           <h2>Digital signal processing textbook for computer science majors. CC Attribution-ShareAlike 4.0 license.</h2>
-        </header>
+<a href="https://zenodo.org/badge/latestdoi/23119912"><img src="https://zenodo.org/badge/23119912.svg" alt="DOI"></a>        </header>
 
         <section id="downloads" class="clearfix">
           <a href="https://github.com/stiber/Signal-Computing/zipball/master" id="download-zip" class="button"><span>Download .zip</span></a>