Edit README to fix grammar and standardize English

Many errors were repeating, due to differences in British and American English. I standardized the README to American English:
- canceled -> cancelled
- canceling -> cancelling
- informations -> information

Remove many instances of passive/awkward voice:
Before: "See the onadd event of how a listener can be registered to be notified when a local notification has been scheduled."
After: "See the onadd event to register a listener to be notified when a local notification has been scheduled."

Misc:
Many it's and its were confused. Conjunctions were sometimes used at inappropriate times.
Fix possessive apostrophes.