imp: Deprecate shallow (#264)

src/shallow.js

@@ -1,11 +1,12 @@
 // @flow
 import * as React from 'react';
 import ShallowRenderer from 'react-test-renderer/shallow'; // eslint-disable-line import/no-extraneous-dependencies
+import { printDeprecationWarning } from './helpers/errors';
 
 /**
  * Renders test component shallowly using react-test-renderer/shallow
  */
-export default function shallow(
+export function shallowInternal(
   instance: ReactTestInstance | React.Element<any>
 ) {
   const renderer = new ShallowRenderer();
@@ -16,3 +17,11 @@ export default function shallow(
     output: renderer.getRenderOutput(),
   };
 }
+
+export default function shallow(
+  instance: ReactTestInstance | React.Element<any>
+) {
+  printDeprecationWarning('shallow');
+
+  return shallowInternal(instance);
+}