docs: add a readme for mac setup

.github/workflows/ci.yml

@@ -50,6 +50,7 @@ jobs:
       - run: |
           mkdir dmg
           cp -a build/CanvasSync.app 'dmg/Canvas Sync.app'
+          mv ./deploy/mac-readme.txt dmg/README.txt
           ln -s /Applications dmg/Applications
           hdiutil create -volname CanvasSync \
             -srcfolder dmg \

deploy/mac-readme.txt

@@ -0,0 +1,18 @@
+Before running Canvas Sync for the first time, open a terminal window
+and run
+
+```
+xattr -c 'Canvas Sync.app'
+```
+
+first. Otherwise there will be an error saying that this app is
+developed by an unidentified developer, but that's just me, the dev.
+
+If you've dragged-and-dropped Canvas Sync into the /Applications
+folder already, simply run
+
+```
+xattr -c '/Applications/Canvas Sync.app'
+```
+
+instead. Enjoy!