Add NDI Input plugin screenshot

include/io-plugins.html

@@ -22,6 +22,11 @@
   IP to a receiver for broadcasting.
 </p>
 
+<figure>
+  <img style="width:50%;" width="50%" src="/images/io-plugins-ndi-input.png" alt="NDI Input plugin loaded as a pre-processing I/O plugin">
+  <figcaption>NDI Input plugin loaded as a pre-processing I/O plugin</figcaption>
+</figure>
+
 <p>
   It's also possible to use the post-processing section to load plugins for room
   correction or signal analysis (VU meters, spectrum analyzers etc.).