[JOSS Review] Improve tests docs

[gbouras13]

Re openjournals/joss-reviews#6062

After running

snakemake --snakefile Snakefile.generate-rawdata -j 8 --use-conda

which completed perfectly, the instructions say:

"To initialise your own workflow, copy ./Snakefile and ./config.yml to a new directory"

However, there isn't a Snakefile or config.yml file in the tests directory. I assume that you need to start a new project as per the main documentation - it would be good to make this clearer.

George