Usage around authoring branches is unclear

[shreve]

Hey there,

The help string for the course parameter of the resource_limits commands includes some important info about where to find the needed id. However, when a course has multiple iterations, this command will opaquely apply the commands to one specific branch and not inform you of which one. For a long time, this seemed like a failure of the tool. Today I met with a Coursera engineer who directed me to figure out which authoring branch I really meant and use that branch's ID instead with this endpoint:

https://api.coursera.org/api/authoringBranchProperties.v1?q=course&courseId={course_id}&fields=properties

I think it would be very helpful to reference information about this endpoint and authoring branches in the help as well.

[josephl310]

Thanks @shreve! It would definitely be helpful to have that info in the README. Created a PR here to address this: #39