Adjust README for Flutter Android SDK setup

[LemonSpike]

The README.md suggests the following to be added to the shell config to enable pidcat:

export PATH=$PATH:<path to Android SDK>/platform-tools
export PATH=$PATH:<path to Android SDK>/tools

This step is unnecessary for machines that have the ANDROID_SDK_ROOT environment variable set up in the shell config for Flutter developers. We can modify the documentation to account for this since Android Studio also supports Flutter development.