docs: refine README

[dominikburri]

• test execution description should be before the DAG run. The test downloads additional files. How much disk space, approximately?
• config.yaml can possibly include the same test files as the test run within test/test_singularity_execution/. Alternative: download the additional files as for the test run.
• write how long the test will run on local and/or cluster execution.

[dominikburri]

after executing bash test/test_singularity_execution/test_local.sh without cleanup, i.e. commenting it out:

cleanup () {
    rc=$?
#    rm -rf .cache/
#    rm -rf .config/
#    rm -rf .snakemake/
#    rm -rf logs/
#    rm -rf results/
    cd $user_dir
    echo "Exit status: $rc"
}

the RCRUNCH total directory size is: 4968M (with du -BM). This contains the test files plus the singularity images.

The test took approx. 8 minutes on cluster with local-singularity profile with the default 4 cores.