README.md uninstall guide suggests deleting gstack state

[dgrant]

# 4. Remove global state
rm -rf ~/.gstack

This should not be part of the uninstall. It should be part of a separate "purge gstack data" or something.

[mvanhorn]

Submitted a fix in #1143 - keeps the numbered block intact but labels it as the --keep-state equivalent, then moves the rm -rf ~/.gstack into a separate "Also purge gstack data (optional)" subsection. No script change; just documents the split that the actual uninstall script already supports via --keep-state.