docs: add example of using GNU parallel to run tasks in parallel

[amarao]

I found amazing synergy between GNU Parallel and just (basically, use parallel in --shebang mode), adding example into examples/.

[casey]

This is great! I think we should actually add this to the readme, since it seems super useful.

A few things:

• There's a num_cpus() function, which can be used to set the number of jobs appropriately.
• I think it's a good idea to use long flags instead of short flags, so --jobs instead of -j.
• With --ungroup, parallel prints job output right away, instead of waiting for the job to finish, which lets you see when jobs start up.
• /usr/bin/env will run parellel regardless of where it is.
• I think we should call the recipe parallel, since churn is a bit uninformative.

With these changes, the recipe looks like this:

parallel:
  #!/usr/bin/env parallel --shebang --ungroup --jobs {{ num_cpus() }}
  echo task 1 start; sleep 1; echo task 1 done
  echo task 2 start; sleep 2; echo task 2 done
  echo task 3 start; sleep 3; echo task 3 done
  echo task 4 start; sleep 4; echo task 4 done

[amarao]

I've updated the example.

[amarao]

Oh, and I found how to force env and parallel to work together in shebang mode.

Thanks for feedback.

[casey]

I moved this into the readme, since I think it'll be more discoverable there. Thanks for the PR!