Update README.md for recent updates.

[chetu-ko]

Hello,

I'm encountering issues with the flutter_unity_widget package example. Specifically, I'm having trouble with the steps related to adding the NDK directory path and creating a new activity. The provided instructions in the README seem ambiguous or incomplete. Please verify if these steps are correct and provide more detailed guidance. If the issue is unrelated to the package, feel free to close this ticket.

NDK ISSUE
Adding this line encountered an error reference here.

ACTIVITY IN ANDROIDMENIFEST
Adding below is encountering the error reference here.

IMAGE IS NOT SHOWING
The information below seems to be very little. How exactly does it need to be implemented in the unity project to get a response object from the unity.