Add description of Decision Point Scope and Decision Tree Scope to Customization Guidance #239

See Discussion #223 for background detail.

• Add a subsection to /doc/md_src_files/060_decision-trees.md under Tree Construction and Customization Guidance to explain how customization might be shared among stakeholder communities
  • touch on both the Decision Point Scope topic as well as the Decision Tree Scope as outlined in this comment in Provide better summary for what is the same and what is different across organizations #223.