Add contents to SETUP.md
Add contents to SETUP.md

To run the tests was not obvious for a beginner, and required
digging into the issues on the exercism/xlfe repo to determine
how to run the tests, as there is not a clear and explicit
path to do so in the README.md unlike other languages.

Add the recommendation for using `make` and running `make test`
to run the tests for the exercise folder the user is in.

exercism/xlfe,lfex/xlfe