Request documentation update to reference latest learn cli commands

[tsiddique]

While doing some research on the Learn CLI and creating content on Learn, I noticed that the README instructions might be out of date and that an alias exists that doesn't seem to be in use.

Specifically, I noticed that the command "learn new" no longer worked, "learn guide" replaced it, and there is a third command "learn walkthrough" that appears to do the same thing as "learn guide." I also noticed that "learn guide" is an alias for walkthrough command but not listed in the output when I run the "learn help" command.

https://github.com/gSchool/glearn-cli/blob/cff7d28fcb8c9e66b220f553865056c564fe9214/app/cmd/guide.go

It looks like if a code change is applied to the file above and the readme is updated to reflect the new command (learn walkthrough) then things should be good to go.