Add gzip size badge

[OliverJAsh]

Fixes #[issue number].

Changes proposed:

Upgrade Path (for changed or removed APIs):

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage increased (+0.4%) to 81.053% when pulling d5bdc51 on OliverJAsh:patch-1 into 37a6ceb on reactjs:master.