Include instructions for specifying merge_run_ids when merging projects in external instructions

[allyhawkins]

In #708, we are adding a new optional argument to the merge workflow, merge_run_ids. The project ID is the only required input, but if desired, users can specify a set of run, sample, or library IDs to include when merging a project. This is optional, so we don't need to include it in the external instructions, but I'm filing this issue now in case we have time/ want to do this in the future.