Update README.md

add ,
Merge pull request #57 from wxianfeng/patch-1

Update README.md
Added support for content-available in Grocer::Notification
Got rid of magic number and made it clearer that the value passed to content_available= isn't used.
Distinguish between truthy and falsy in content_available.
Should test for absence of key in hash, not for its value being nil.
Cleaned up falsy content_available test.
Updated docs for content_available.
Updated docs for content_available to match the other keys in the payload.
Merge pull request #58 from somezack/add_content_available

Added support for content-available
Add Ruby 2.0.0 for Travis CI
DRY-up setting of the content-available field

A NewsstandNotification just merges in the value and lets the base
Notification do the work.
Consistent use of string literals
Prepare for the v0.4.0 release
Bump to version 0.4.0
Failing test case for issue with content-available in using Grocer::Server.
Clean bad payload keys when reading Notifications

NotificationReader will sanitize payload keys, replaceing '-' with '_'
before passing the payload on to the Notification object.

Closes #59
Bump to version 0.4.1
tiny patch up based on @stevenharman's review request
Merge pull request #60 from daveworth/use_each_with_object

tiny patch up based on @stevenharman's review request