Add helpful information to the SETUP.md

[kytrinyx]

The contents of the SETUP.md file gets included in
the README.md that gets delivered when a user runs the exercism fetch
command from their terminal.

At the very minimum, it should contain a link to the relevant
language-specific documentation on
help.exercism.io.

It would also be useful to explain in a generic way how to run the tests.
Remember that this file will be included with all the problems, so it gets
confusing if we refer to specific problems or files.

Some languages have very particular needs in terms of the solution: nested
directories, specific files, etc. If this is the case here, then it would be
useful to explain what is expected.

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Thanks, @tejasbubane for suggesting that we add this documentation everywhere.
See exercism.io#2198.