[GB] add example usage, fix #2414

docs/gekko-broker/introduction.md

@@ -50,6 +50,78 @@ TODO:
   - Stop
   - If Touched (stop but opposite direction)
 
+### Example
+
+Set up a Gekko Broker instance:
+
+    // from the gekko repo (make sure you have deps installed
+    // inside the exchange folder).
+    const Broker = require('../gekko/exchange/GekkoBroker');
+    // or from NPM
+    // const Broker = require('gekko-broker');
+
+    const binance = new Broker({
+      currency: 'USDT',
+      asset: 'BTC',
+      private: true,
+
+      exchange: 'binance',
+      key: 'x', // add your API key
+      secret: 'y' // add your API secret
+    });
+
+Now we have an instance that can create a [sticky order](./sticky_order.md):
+
+    const type = 'sticky';
+    const side = 'buy';
+    const amount = 1;
+
+    const order = binance.createOrder(type, side, amount);
+
+    order.on('statusChange', s => console.log(now(), 'new status', s));
+    order.on('fill', s => console.log(now(), 'filled', s));
+    order.on('error', s => console.log(now(), 'error!', e));
+    order.on('completed', a => {
+      console.log(new Date, 'completed!');
+      order.createSummary((err, s) => {
+        console.log(new Date, 'summary:');
+        console.log(JSON.stringify(s, null, 2));
+      });
+    });
+
+This one doesn't have an upper limit price for what it will buy at. It will stick it's bid offer at BBO until it's filled. If you have a limit in mind you can specify it when creating, do this instead:
+
+    const order = binance.createOrder(type, side, amount, { limit: 100 });
+
+It will never offer to buy for more than 100, even if the BBO is above 100 (the bid will end up deep in the book).
+
+At any point in time you can change the limit (or the amount), for example:
+
+    order.moveLimit(120);
+
+Running the above example (without setting a limit and moving it) will yield:
+
+    root@foxtail:~/gb/nusadua# node b
+    2018-07-29 03:46:02 new status SUBMITTED
+    2018-07-29 03:46:02 new status OPEN
+    2018-07-29 03:46:04 filled 1
+    2018-07-29 03:46:04 new status FILLED
+    2018-07-29T03:46:04.127Z 'completed!'
+    2018-07-29T03:46:04.358Z 'summary:'
+    {
+      "price": 0.0017479,
+      "amount": 1,
+      "date": "2018-07-29T03:46:02.576Z",
+      "side": "buy",
+      "orders": 1,
+      "fees": {
+        "BNB": 0.00075
+      },
+      "feePercent": 0.075
+    }
+
+NOTE: not all status changes are documented and more events are planned but not implemented.
+
 ### TODO
 
 - finish all exchange integrations that gekko supports