Model card: clear the old unpack directory when upgrading

A second version unpacks into the same directory as the first, and macOS kills a process that mixes two builds -- `Killed: 9`, with nothing to say why. The install now clears it, and both that and the PATH case are written out.

README.md

@@ -63,6 +63,9 @@ the wait is the SSD's, not the model's.
 ## Install
 
 ```bash
+# upgrading? clear the previous version's unpack directory first
+rm -rf ~/.libra/cache/onefile/gbx_lm
+
 curl -fL -o gbx_lm-darwin-arm64.tar.gz 'https://github.com/GreenBitAI/gbx-lm/releases/latest/download/gbx_lm-darwin-arm64.tar.gz' \
   && tar -xzf gbx_lm-darwin-arm64.tar.gz gbx_lm \
   && mkdir -p "$HOME/.local/bin" \
@@ -72,9 +75,18 @@ curl -fL -o gbx_lm-darwin-arm64.tar.gz 'https://github.com/GreenBitAI/gbx-lm/rel
 gbx_lm -h
 ```
 
-`command not found` means `$HOME/.local/bin` is not on your `PATH`: add it, or
-call the binary by its full path. The build is signed with a Developer ID and
-notarised, so macOS runs it without the usual detour for a downloaded binary.
+The build is signed with a Developer ID and notarised, so macOS runs it without
+the usual detour for a downloaded binary.
+
+**`command not found`** -- `$HOME/.local/bin` is not on your `PATH`:
+
+```bash
+echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.zshrc && source ~/.zshrc     # zsh
+echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.bash_profile && source ~/.bash_profile   # bash
+```
+
+**`Killed: 9`** -- a previous version's files are still in the unpack directory,
+and macOS refuses to mix two builds. Run the `rm -rf` line above, then try again.
 
 ## Run