Update README.md to include git in requirements

[jgreever]

It seems very small, and not really a problem, most people I'm sure would figure it out, but it might be helpful to add the following to the README.md file under:

For building on a fresh Raspbian Stretch or Buster install, these steps should be run:

sudo apt-get install git

I imagine it might not be a bad idea to ensure each Distro listed includes git in the requirements, even though users can download the code as a zip file, but the README.md does list using git to grab the code.

For downloading the code, use these commands:
git clone https://github.com/FD-/RPiPlay.git
cd RPiPlay

Thank you for your awesome work on this project!