add r8 android specific requirements to getting started and sample projects

plz read the last couple of comments on this thread #255 (comment 94479)

The front page documentation, as well as the "getting started", which contains other platform related requiremtns do not mention the information in https://code.videolan.org/videolan/LibVLCSharp/-/blob/3.x/docs/android.md.

Furthermore I don't think the sample projects contain this r8 config as well.