Added manual and script for PowerShell autocomplete #1139
What type of PR is this?
What this PR does / why we need it:
This adds documentation and script to enable autocomplete on both
Special notes for your reviewer:
Not sure how to automate testing
Following instructions added to the manual in a PowerShell environment
Oh, @kirinnee the docs tests are failing b/c you're missing a heading. Check the logs here => https://github.com/urfave/cli/pull/1139/checks?check_run_id=683967954