Redme file have instructions for old unity version

[guyluz11]

I have just started going through the instructions in the readme and there are some parts that can be explained better and others that have old info (or missing).

I am using editor version 2021.3.16f1 LTS Personal and Unity Hub version 3.4.1. The demo is built on 2019.4.40f1 LTS

• Can we upload FlutterUnityPackage.unitypackage to the unity package store?
• NDK section needs to mention that you need to search the location inside the Unity editor and not the android studio editor as it is a little bit confusing.
• NDK path explanation is incorrect in the current Unity version, it does not exist under Preferences -> External Tools. It moved to Edit -> Preferences -> External Tools.
• Under Steps number 3 is incorrect in the current unity version, it does not exist under Configuration section but under Other Settings tab and then Configuration section.
• Under Steps number 3 the image is old and x86 got changed to x86 (Chrome OS), not sure if this is important as the instructions are talking only about ARMv7 and ARM64
  
• "Be sure you have at least one scene added to your build" more explanation or images where to check can be helpful.
• Clear instructions for importing FlutterUnityPackage.unitypackage correctly because there are a lot of issues that are getting opened on it.