Update README for testnet

Author: robsantini

README.md

@@ -50,7 +50,9 @@ $api = new Binance\API();
 $api = new Binance\API( "somefile.json" );
 // 3. config by specifying api key and secret
 $api = new Binance\API("<api key>","<secret>");
-// 4. Rate Limiting Support
+// 4. config by specifying api key, api secret and testnet flag. By default the testnet is disabled
+$api = new Binance\API("<testnet api key>","<testnet secret>", true);
+// 5. Rate Limiting Support
 $api = new Binance\RateLimiter(new Binance\API());
 ```
 See [additional options](https://github.com/jaggedsoft/php-binance-api/#config-file-in-home-directory) for more options and help installing on Windows
@@ -1343,6 +1345,19 @@ cat >  ~/.config/jaggedsoft/php-binance-api.json << EOF
 EOF
 ```
 
+#### Config file in home directory to operate on testnet
+Testnet have its own credentials, see the [testnet documentation page](https://testnet.binance.vision/) for more details.
+```bash
+mkdir -vp ~/.config/jaggedsoft/
+cat >  ~/.config/jaggedsoft/php-binance-api.json << EOF
+{
+    "api-key": "<testnet api key>",
+    "api-secret": "<testnet secret>",
+    "use-testnet": true
+}
+EOF
+```
+
 #### Config file in home directory with curl options
 ```bash
 mkdir -vp ~/.config/jaggedsoft/