Add uninstall directions for flyctl

[catflydotio]

Homebrew: brew uninstall (no deps?) (if any, do brew autoremove too?)

If installed with curl, it's just a matter of removing the .fly dir (and removing any env vars and paths added manually to .bashrc or .zshrc)?

Powershell the same?

[michaeldwan]

Homebrew: brew uninstall (no deps?) (if any, do brew autoremove too?)

brew uninstall -- no deps here

If installed with curl, it's just a matter of removing the .fly dir (and removing any env vars and paths added manually to .bashrc or .zshrc)?

correct

Powershell the same?

correct