Summary:
Clearly state that the `getSize()` does not work for static image resources because it is assumed the developer knows their static image resource size (the reason I got from a comment on a github issue by a react native contributor).
This missing documentation cost me about 30 minutes to figure out why it didn't work for my static image resource and so I decided to update the documentation 💃
Closes https://github.com/facebook/react-native/pull/11645
Differential Revision: D4533463
Pulled By: lacker
fbshipit-source-id: 70e175ea30a5540c8a9f2a0c040585c711d82ac3
0150bc76eb