docs: updated docs for always-return

README.md

@@ -53,6 +53,8 @@ Ensure that inside a `then()` you make sure to `return` a new promise or value.
 See http://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html (rule #5)
 for more info on why that's a good idea.
 
+We also allow someone to `throw` inside a `then()` which is essentially the same as `return Promise.reject()`.
+
 #### Valid
 
 ```js