Revert "contentInset and contentOffset should be the same for the placeholder"

This reverts commit 6a84ea70235c2b70248e7d8c62b0cd7938185019.

Removes sync of `contentInset` and `contentOffset` because it leads to layout issues.
Insets/offsets apply to child views' positioning so the placeholder text
view was positioned wrongly.

Resolves #21, closes #22.
upgrading readme info about supported SDKs