[doc] Add screenshots for new gruvbox-light themes

see #256

README.md

@@ -192,6 +192,10 @@ Gruvbox Powerline (`-t gruvbox-powerline`) (contributed by [@TheEdgeOfRage](http
 
 ![Gruvbox Powerline](https://github.com/tobi-wan-kenobi/bumblebee-status/blob/master/screenshots/themes/powerline-gruvbox.png)
 
+Gruvbox Powerline Light (`-t gruvbox-powerline-light`) (contributed by [feed00m](https://github.com/freed00m)):
+
+![Gruvbox Powerline Light](https://github.com/tobi-wan-kenobi/bumblebee-status/blob/master/screenshots/themes/gruvbox-powerline-light.png)
+
 Solarized Powerline (`-t solarized-powerline`):
 
 ![Solarized Powerline](https://github.com/tobi-wan-kenobi/bumblebee-status/blob/master/screenshots/themes/powerline-solarized.png)
@@ -200,6 +204,10 @@ Gruvbox (`-t gruvbox`):
 
 ![Gruvbox](https://github.com/tobi-wan-kenobi/bumblebee-status/blob/master/screenshots/themes/gruvbox.png)
 
+Gruvbox Light (`-t gruvbox-light`) (contributed by [feed00m](https://github.com/freed00m)):
+
+![Gruvbox Light](https://github.com/tobi-wan-kenobi/bumblebee-status/blob/master/screenshots/themes/gruvbox-light.png)
+
 Solarized (`-t solarized`):
 
 ![Solarized](https://github.com/tobi-wan-kenobi/bumblebee-status/blob/master/screenshots/themes/solarized.png)