Add 'post JSON' example to readme (#173)

README.md

@@ -125,6 +125,20 @@ fetch('http://httpbin.org/post', { method: 'POST', body: stream })
 		console.log(json);
 	});
 
+// post with JSON
+
+var body = { a: 1 };
+fetch('http://httpbin.org/post', { 
+	method: 'POST',
+	body:    JSON.stringify(body),
+	headers: { 'Content-Type': 'application/json' },
+})
+	.then(function(res) {
+		return res.json();
+	}).then(function(json) {
+		console.log(json);
+	});
+
 // post with form-data (detect multipart)
 
 var FormData = require('form-data');