Add README describing the package and usage

Author: CodeMangler

README.md

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+# Google Maps Javascript API Stub
+
+Google Maps Javascript API is exposed under the `window.google` namespace, and has an object structure that's fairly involved.
+This makes testing of components and other UI logic that depend on the Maps API a bit inconvenient, since you would inevitably have to stub out quite a few of the Maps API objects and method calls in your tests, even for fairly trivial usages.
+It usually ends up adding noise to your tests, and obscuring their intent.
+
+This package stubs out the `window.google` namespace, and all documented objects and methods of the Maps API under this namespace.
+All of the stubs are `noop`s and usually return an empty object, or an object with a sensible structure as appropriate.
+Importing this package into your tests will let you focus on testing your component and UI logic without having to worry about details of the Maps API.
+In addition, you can set up explicit expectations on specific Google Maps API calls when you are testing interactions with the Maps API.
+
+Initial versions of this package stub out significant portions of the documented Maps API.
+Eventually this package will be versioned in lockstep with the Maps API versions (i.e. importing version `X` of this package will stub out version `X` of the Maps API).
+
+## Usage
+- Install the package
+
+  `npm install --save-dev googlemaps-js-api-stub`
+
+  OR
+  
+  `yarn add --dev googlemaps-js-api-stub`
+  
+- Import `googlemaps-js-api-stub` into your test
+  
+  `import 'googlemaps-js-api-stub'`
+  
+  OR
+  
+  `require 'googlemaps-js-api-stub'`
+  
+- Optionally, set expectations on specific API calls as required
+
+```javascript
+sinon.stub(window.google.Map, 'setZoom');
+expect(window.google.Map.setZoom).to.have.been.called
+```