docs(usage): add custom form id

Author: jsun969

README.md

@@ -20,9 +20,11 @@
 npm install @hookform/devtools -D
 ```
 
-3. Add `<Devtool control={control} />` in your code  
-   _This won't show devtools component anymore! Just for extension detect your form_
-4. Enjoy!
+3. Add [DevTool component](https://github.com/react-hook-form/devtools#quickstart) in your code  
+   _This won't show devtool component anymore! Just for extension detect your form_
+4. (Optional) Add custom form id: `<DevTool id="YourFormId" control={control} />`  
+   _If you don't add it manually, it will be auto generated by uuid_
+5. Enjoy!
 
 ## Development