Add best practices for dummy atoms

[fjclark]

Proposed Change

Add a brief section in "What simulation protocol should I choose" on best practices for dummy atoms as detailed here.

Proposed Authors and Reviewers

Please confirm (before the end of January) that you are happy to author/ review and state when you're available. Authors, please say roughly when you plan to contribute the changes (no later than the end of May).

Proposed authors: @wiederm and/ or @sboresch
Proposed reviewer: Volunteer wanted

[sboresch]

Sure, I'll coordinate with @wiederm!