docs(*): Add README

Added README with basic usage example

README.md

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+# HealthGraphQL
+
+A GraphQL wrapper around the Health Graph, to support reading and writing data.
+
+> Currently supports reading profile information & fitness activities, and reading & writing strength training activities.
+
+## Usage
+```javascript
+var express = require('express');
+var healthgraphql = require('healthgraphql');
+var app = express();
+
+app.use(healthgraphql({
+	graphiql: true,
+	getAccessToken: function (req) {
+		// return access token here
+	}
+}));
+
+app.listen(3000);
+```
+
+### Options
+`graphiql` - Boolean. Enable or disable the graphiql interface.  
+`getAccessToken` - Function (request). Should return the access token to use to access the Health Graph.