Adjust CONTRIBUTING.md to make it more appropriate for a new lesson

[tobyhodges]

The template CONTRIBUTING.md includes some information, guidance, and links that are not really appropriate for a brand new lesson being created from a template. E.g. in Where to Contribute the current version points to the SWC shell-novice lesson & repository. It also links to the lesson-example repository, and that should be updated to point to a Workbench equivalent.

[zkamvar]

I wonder if this should also be the case for {sandpaper} as well since it will not generally create already established lessons.

[tobyhodges]

Yes, you are probably right. Creating new template text for this is on my list of projects for an R&R day soon.

[zkamvar]

This has been partially addressed in carpentries/sandpaper#407 thanks to @jcolomb

[zkamvar]

I realise that this is not exactly addressed, so I'm going to reopen it.