Add docs about backends to tfjs-core readme (tensorflow#3382)

DOC

Author: pvaneck

tfjs-backend-cpu/README.md

@@ -10,7 +10,7 @@ Note: this backend is included by default in `@tensorflow/tfjs`.
 
 ```js
 // Import @tensorflow/tfjs-core
-import * as tf from '@tensorflow/tfjs';
+import * as tf from '@tensorflow/tfjs-core';
 // Adds the CPU backend to the global backend registry.
 import '@tensorflow/tfjs-backend-cpu';
 ```

tfjs-backend-webgl/README.md

@@ -10,7 +10,7 @@ Note: this backend is included by default in `@tensorflow/tfjs`.
 
 ```js
 // Import @tensorflow/tfjs-core
-import * as tf from '@tensorflow/tfjs';
+import * as tf from '@tensorflow/tfjs-core';
 // Adds the WebGL backend to the global backend registry.
 import '@tensorflow/tfjs-backend-webgl';
 ```

tfjs-core/README.md

@@ -31,6 +31,10 @@ import * as tfc from '@tensorflow/tfjs-core';
 // No Layers API.
 ```
 
+**Note**: If you are only importing the Core API, you also need to import a
+backend (e.g., [tfjs-backend-cpu](/tfjs-backend-cpu),
+[tfjs-backend-webgl](/tfjs-backend-webgl), [tfjs-backend-wasm](/tfjs-backend-wasm)).
+
 For info about development, check out [DEVELOPMENT.md](/DEVELOPMENT.md).
 
 ## For more information