add README

Author: mavajee

README.md

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+`react-nullable-transition` is simple wrap component for using `react-transition-group` with jsx stataments (like vue transition).
+
+For example you can create `OpacityTransition` component based on this package and use it like this:
+
+```jsx
+<OpacityTransition>
+  {show && <Square />}
+</OpacityTransition>
+```
+
+See full example [here](https://github.com/mavajee/react-nullable-transition/tree/master/example).