Document Homebrew formula to Cask migration (#397)

Update Mac installation instructions to use Cask and add migration guide for existing formula users. Addresses #390 by documenting the upgrade path from deprecated formula to Cask version.

Author: owenthereal

README.md

@@ -15,9 +15,23 @@ This is a [blog post](https://owenou.com/upterm) to describe Upterm in depth.
 ### Mac
 
 ```console
-brew install owenthereal/upterm/upterm
+brew install --cask owenthereal/upterm/upterm
 ```
 
+#### Migrating from Formula to Cask
+
+If you previously installed upterm using the Homebrew formula (without `--cask`), you'll need to migrate to the Cask version:
+
+```console
+# Uninstall the old formula version
+brew uninstall upterm
+
+# Install the new Cask version
+brew install --cask owenthereal/upterm/upterm
+```
+
+**Note:** Running `brew upgrade` with the old formula installed will fail with an error. Follow the migration steps above to resolve this.
+
 ### Standalone
 
 `upterm` can be easily installed as an executable. Download the latest [compiled binaries](https://github.com/owenthereal/upterm/releases) and put it in your executable path.