Add bucket check to db.View in readme.md
Add bucket check to db.View in readme.md

Add 'Assume bucket exists and has keys' comments on db.View calls
accessing bucket keys in examples of db.View in readme.md code examples.
Fixes #484

etcd-io/bbolt,boltdb/bolt,li-ang/bolt,chlorm-forks/bolt,benbjohnson/bolt,tv42/bolt,btcsuite/bolt