docs: explain permission usage (fixes #8)

README.md

@@ -4,4 +4,11 @@ Store coupons for websites. No external websites involved. Just a specialized st
 
 ## Translations
 
-Translations can be contributed on Transifex: https://www.transifex.com/freaktechnik/coupon-booklet/
+Translations can be contributed on Transifex: https://www.transifex.com/freaktechnik/coupon-booklet/
+
+
+## Permissions
+
+- _Input data to the clipboard_: Allows the extension to copy the coupon code to the clipboard when you command it to.
+- _Access browser tabs_: Used to show a count of available coupons for the current tab on the toolbar button; highlights the current page when opening the popup and allows auto-filling the current website when adding a coupon
+- _Store unlimited amount of client-side data_: Makes sure your coupon codes survive the hardest of times