R Dev Container: document opening R terminal with VS Code command palette

[hturner]

This is issue 44 on the R Dev Container repo.

The relevant documentation bits of the documentation are now part of the online docs rather than the README:

• starting R - users may skip the previous "Running R" section
• editing source code (askYesNo example)

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

[hturner]

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