Add supported node version to documentation

Hi!
Since you are using node-sass `4.13.1` and this version is not compatible with the latest node version (16) it can be useful to state in the documentation what is the required node version (I found it's working with node 15).

Thanks!

Inspect the repository documentation to find where the required Node.js version is described or should be added. Confirm the version compatible with node-sass 4.13.1, update the documentation with that requirement, and verify the resulting guidance is clear.

Written by the indexing model from the issue text.