[documentation] add banner image

Author: zombieFox

README.md

@@ -1,5 +1,7 @@
 # nightTab
-New-tab-page/Start-page for all browsers
+A neutral new tab page accented with a chosen colour. Customise the layout, style, background and bookmarks in nightTab -- a custom start page.
+
+[![nightTab Demo](images/banner-1400-560.png)](https://zombiefox.github.io/nightTab/)
 
 ## [See it in action](https://zombiefox.github.io/nightTab/)