Add required versions of yarn and node on the README

[jorgearuv]

While I was setting up the project I noticed this project doesn't play along with Yarn 1.22.*, I had to install 1.9.4. The same with Node, I had v14 installed and had to install v10.

If you think this is ok I can send a PR for it.

[RSeidelsohn]

I had issues running the initial yarn with node v15.5.1 and yarn 1.22.4, but after a simple yarn upgrade everything runs smoothly. So you first could try a yarn upgrade.