Add instructions for including esp-idf driver component

Author: vickash

README.md

@@ -10,6 +10,15 @@ In `esp32_build_config.rb`, at the end of the `MRuby::CrossBuild` block, add the
   conf.gem :github => 'mruby-esp32/mruby-esp32-gpio'
 ```
 
+In `CMakeLists.txt`, make sure the line `PRIV_REQUIRES` includes the `driver` component. Add it if needed:
+
+```
+  add_prebuilt_library(
+    libmruby ${LIBMRUBY_FILE}
+    PRIV_REQUIRES esp_wifi esp_hw_support esp_rom mqtt driver 
+  )
+````
+
 ## Example
 ```ruby
 include ESP32::GPIO