Add README section about getting request latency information. (#8)

Author: danielmai

README.md

@@ -23,6 +23,7 @@ and understand how to run and work with Dgraph.
   - [Run a mutation](#run-a-mutation)
   - [Run a query](#run-a-query)
   - [Commit a transaction](#commit-a-transaction)
+  - [Check request latency](#check-request-latency)
   - [Debug mode](#debug-mode)
 - [Development](#development)
   - [Building the source](#building-the-source)
@@ -234,6 +235,29 @@ try {
 }
 ```
 
+### Check request latency
+
+To see the server latency information for requests, check the
+`extensions.server_latency` field from the Response object for queries or from
+the Assigned object for mutations. These latencies show the amount of time the
+Dgraph server took to process the entire request. It does not consider the time
+over the network for the request to reach back to the client.
+
+```js
+// queries
+const res = await txn.queryWithVars(query, vars);
+console.log(res.extensions.server_latency);
+// { parsing_ns: 29478,
+//  processing_ns: 44540975,
+//  encoding_ns: 868178 }
+
+// mutations
+const assigned = await txn.mutate({ setJson: p });
+console.log(assigned.extensions.server_latency);
+// { parsing_ns: 132207,
+//   processing_ns: 84100996 }
+```
+
 ### Debug mode
 
 Debug mode can be used to print helpful debug messages while performing alters,