Add workflow examples

[espenhgn]

Fixes #42

Changes proposed in this pull request:

• Add "hello world" workflow examples for WDL, Snakemake, Nextflow

Before submitting

• I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
• I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• If this PR fixes a bug, I've added a test that will fail without my fix.
• If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.