Add 'a' keyboard shortcut to add-feed

* Used the BIG_FREE_READER shortcut of 'a' to add-feed.
* I only added the en.yml localization. (I wasn't sure how you went about localizing the other keyboard shortcuts)
* I didn't see any tests for shortcuts, so didn't add one, but did test locally. (If there is a place for me to write a test, let me know and I will dive in)

> This is my first real PR and OSS contribution, so please point me in the right direction if there is something I can change / improve.