docs: Update readme to link to update_prod_branches_to_main.py (#971)

[drstrangelooker]

• Create simple_schedule_plan.py

Many people have come on chat about creating schedules with the sdk and I just wanted to create a barebones example, so we can direct them to a public example.

• Update README.md

Updated readme to point at simple_schedule_plan example

• Update README.md

Update readme to link to new example of adding users to a group from a CSV.

• updated readme to point at script which updates prod branches touse main rather than master

Co-authored-by: Joel Dodge joeldodge@google.com
Co-authored-by: Dr. Strangelove drstrangelove@google.com

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