Update README.md

Add limitations of WeakMap shim.
weakmap is lowercase
Merge pull request #4 from josh/patch-1

weakmap is lowercase
Adding a test suite for WeakMap.
Merge pull request #5 from markfinger/add-test-suite

WeakMap test suite
Add WeakMap.has and conform WeakMap.delete to spec
Merge remote-tracking branch 'upstream/master'
1.0.0