adding API key requirement for Google Maps
diff --git a/README.md b/README.md
index 130d427..69fe59e 100644
--- a/README.md
+++ b/README.md
@@ -3,11 +3,24 @@
This visualization renders Global Navigation Satellite System (GNSS) position
-time series data on top of an interactive map.
+time series data on top of an interactive Google Map.
By manipulating the magnification level and the timespan, you can see how the
Earth has moved over time.
+## Setup
+
+To use Google Maps, you need a browser API key.
+Follow these instructions to
+[Get a Key/Authentication](https://developers.google.com/maps/documentation/javascript/get-api-key#key).
+
+Once you have your API key, open up the `index.html` file and find this line:
+```js
+var apiKey = '';
+```
+
+Paste in your API key and save the file.
+
## Usage
To use this visualization, first you'll need to produce or acquire a GNSS data
diff --git a/index.html b/index.html
index e7089ac..a5b091f 100644
--- a/index.html
+++ b/index.html
@@ -7,7 +7,22 @@
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=RobotoDraft:400,500,700,400italic">
<link rel="stylesheet" href="css/vis.css">
-<script src="https://maps.google.com/maps/api/js"></script>
+<script>
+/**
+ * The Geodetic Velocities visualization makes use of the Google Maps API to
+ * provide an interactive map for exploring the data. To use the Google Maps
+ * API, you need an API key.
+ *
+ * Instructions for getting a key can be found here:
+ * https://developers.google.com/maps/documentation/javascript/get-api-key#key
+ *
+ * Once you have your key, paste it into the variable below.
+ */
+var apiKey = '';
+
+document.write('<script src="https://maps.google.com/maps/api/js?key=' +
+ apiKey + '"></' + 'script>');
+</script>
</head>
<body>
<script src="third_party/js/d3/d3-3.5.11.js"></script>
