update readme with requirements

better_figures_and_images doesn't make it clear in the readme what
requirements are required.

better_figures_and_images/readme.rst

@@ -1,3 +1,8 @@
+Requirements
+------------
+
+* pip install pillow bs4
+
 Summary
 ===========
 
@@ -47,4 +52,4 @@ or this, if RESPONSIVE_IMAGES = True:
 	        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
 	        tempor incididunt ut labore et dolore magna aliqua.
 	    </div>
-	</div>
+	</div>