Add note about homebrew deps for debian build

Fixes: https://github.com/webtorrent/webtorrent-desktop/issues/1689

README.md

@@ -158,6 +158,11 @@ The Mac app can only be packaged from **macOS**.
 
 The Linux app can be packaged from **any** platform.
 
+If packaging from Mac, install system dependencies with Homebrew by running:
+
+```
+npm run install-system-deps
+```
 
 #### Recommended readings to start working in the app
 

package.json

@@ -100,6 +100,7 @@
     "build": "buble src --output build",
     "clean": "node ./bin/clean.js",
     "gh-release": "gh-release",
+    "install-system-deps": "brew install fakeroot dpkg",
     "open-config": "node ./bin/open-config.js",
     "package": "node ./bin/package.js",
     "prepublish": "npm run build",