Minor instruction improvements for Singularity example
- Add info on singulars group to README
- Rename "Prerequisites" to "Things to Adjust" and move it up underneath "Things to Note"
- Add the need to adjust the Definition file and the clone call to README
- Link singularity docs on how to write Definition files
- Add a comment in
singularity.shto encourage users to change the clone call
Fixes #2 (closed). CC @m.wierzba