R Dev Container: document how to view changes to the SVN repo

[hturner]

This is 86 on the R Dev Container repo.

The documentation might be added into the editing source code section.

The source code for this is in contribution_workflow.md - please make a PR to the devel branch.

In order to get screen shots, you will need to follow the instructions to start a codespace and build R, or collaborate with someone who has this already set up.