docs: example of cookie handling

fixes #79

README.md

@@ -146,7 +146,7 @@ var options = {
         some: 'payload' // Will be urlencoded
     },
     headers: {
-        /* 'content-type': 'application/x-www-form-urlencoded' */ // Set automatically
+        /* 'content-type': 'application/x-www-form-urlencoded' */ // Is set automatically
     }
 };
 
@@ -159,6 +159,39 @@ rp(options)
     });
 ```
 
+### Include a cookie
+
+``` js
+var tough = require('tough-cookie');
+
+// Easy creation of the cookie - see tough-cookie docs for details
+let cookie = new tough.Cookie({
+    key: "some_key",
+    value: "some_value",
+    domain: 'api.mydomain.com',
+    httpOnly: true,
+    maxAge: 31536000
+});
+
+// Put cookie in an jar which can be used across multiple requests
+var cookiejar = rp.jar();
+cookiejar.setCookie(cookie, 'https://api.mydomain.com');
+// ...all requests to https://api.mydomain.com will include the cookie
+
+var options = {
+    uri: 'https://api.mydomain.com/...',
+    jar: cookiejar // Tells rp to include cookies in jar that match uri
+};
+
+rp(options)
+    .then(function (body) {
+        // Request succeeded...
+    })
+    .catch(function (err) {
+        // Request failed...
+    });
+```
+
 ### Get the full response instead of just the body
 
 ``` js