Add a README file for Nodes

Summary:
Add a README file explaining the purpose of Nodes and how to
enable it within an app.

Reviewed By: JoelMarcey, lacker

Differential Revision: D4349517

fbshipit-source-id: ec26ebb37039e7c23ecd2cf4b482fa21ca8beeda

Author: Ahmed El-Helw

ReactAndroid/src/main/java/com/facebook/react/flat/README.md

@@ -0,0 +1,49 @@
+# Nodes
+
+Nodes is an experimental, alternate version of
+[UIImplementation](https://github.com/facebook/react-native/blob/master/ReactAndroid/src/main/java/com/facebook/react/uimanager/UIImplementation.java) for ReactNative on Android. It has two main advantages over the existing `UIImplementation`:
+
+1. Support for `overflow:visible` on Android.
+2. More efficient generation of view hierarchies.
+
+The intention is to ultimately replace the existing `UIImplementation` on
+Android with Nodes (after all the issues are ironed out).
+
+## How to test
+
+In a subclass of `ReactNativeHost`, add this:
+
+```java
+@Override
+protected UIImplementationProvider getUIImplementationProvider() {
+  return new FlatUIImplementationProvider();
+}
+```
+
+## How it Works
+
+The existing
+[UIImplementation](https://github.com/facebook/react-native/blob/master/ReactAndroid/src/main/java/com/facebook/react/uimanager/UIImplementation.java) maps all non-layout tags to `View`s (resulting in an almost 1:1 mapping of tags
+to Views, with the exception of some optimizations for layout only tags that
+don't draw content). Nodes, on the other hand, maps react tags to a set of
+`DrawCommand`s. In other words, an `<image>` tag will often be mapped to a
+`Drawable` instead of an `ImageView` and a `<text>` tag will be mapped to a
+`Layout` instead of a `TextView`. This helps flatten the resulting `View`
+hierarchy.
+
+There are situations where `DrawCommand`s are promoted to `View`s:
+
+1. Existing Android components that are wrapped by React Native (for example, 
+`ViewPager`, `ScrollView`, etc).
+2. When using a `View` is more optimal (for example, `opacity`, to avoid
+unnecessary invalidations).
+3. To facilitate the implementation of certain features (accessibility,
+transforms, etc).
+
+This means that existing custom `ViewManager`s should continue to work as they
+did with the existing `UIImplementation`.
+
+## Limitations and Known Issues
+
+- `LayoutAnimation`s are not yet supported
+- `zIndex` is not yet supported