Add brief explanation about app_main() function when arduino used as a esp-idf component (espressif#99)

* Add brief explanation about app_main() function

* Update README.md

The proposed sentence is restated in a more clear way.

* Fix typo

Author: eroniki

README.md

@@ -106,9 +106,11 @@ Linux 32/64, Linux ARM (like Raspberry Pi, BeagleBone, CubieBoard).
             delay(1000);
           }
           ```
+          
         - Else you need to implement ```app_main()``` and call ```initArduino();``` in it.
 
-          Keep in mind that setup() and loop() will not be called in this case
+          Keep in mind that setup() and loop() will not be called in this case.
+          If you plan to base your code on examples provided in [esp-idf](https://github.com/espressif/esp-idf/tree/master/examples), please make sure move the app_main() function in main.cpp from the files in the example.
 
           ```arduino
           //file: main.cpp