Document available channels

[KronosTheLate]

I was going to add the release channel, but I thought it was called latest. It would be very helpful if juliaup add would print the possible channels, as the current solution seems to be to navigate to the GitHub repo and read the readme. The current output is:

C:\Users\Dennis Bal>juliaup add
error: The following required arguments were not provided:
    <CHANNEL>

USAGE:
    juliaup add <CHANNEL>

For more information try --help

[kimlaberinto]

I was just taking a look at some issues and noticed relevant commits for this issue. I wanted to document them here in case someone else takes a look at this.

• juliaup list command was implemented in commit 2c4f102
• juliaup list test was implemented in commit aefa8c8

I'm not sure if the above commits are sufficient.

A repo maintainer can comment as they see fit on how to proceed.