(docs): Add graph setup documentation for MacOS (#146)

Thanks @naistran for instructions, and @leothelocust and @seandavi for
discussion and confirmation

doc/graph_setup.md

@@ -5,7 +5,7 @@ The gist of the setup is setting up a Bash script to trim off the
 `roam://` prefix from the link, causing the desktop application to
 call `emacsclient path/to/org-roam-file.org`.
 
-## Setting Up for Linux
+## Linux
 
 Create a desktop application. I place mine in
 `~/.local/share/applications/roam.desktop`:
@@ -37,3 +37,42 @@ running in your shell:
 ```bash
 xdg-mime default roam.desktop x-scheme-handler/roam
 ```
+
+## Mac OS
+
+One solution to this, recommended in [Issue
+#115](https://github.com/jethrokuan/org-roam/issues/115), is to use
+[Platypus](https://github.com/sveinbjornt/Platypus). Here are the
+instructions for setting up with Platypus and Chrome:
+
+1. Create an executable `launch-emacs.sh` script:
+
+```sh
+#!/usr/bin/env bash
+/usr/local/bin/emacsclient --no-wait "${1#*:}"
+```
+
+2. Install and launch Platypus (with [Homebrew](https://brew.sh/)):
+
+```sh
+brew cask install playtpus
+```
+
+3. Playtpus settings:
+
+- App Name: `OrgRoam`
+- Script Type: `env` and `/usr/bin/env`
+- Script Path: `/path/to/your/launch-emacs.sh`
+- Tick Accept dropped items and click Settings
+- Tick Accept dropped files
+- Tick Register as URI scheme handler
+- Add `roam` as a protocol
+- Create the app
+
+To disable the "confirm" prompt in Chrome, you can also make Chrome
+show a checkbox to tick, so that the `OrgRoam` app will be used
+without confirmation. To do this, run in a shell:
+
+```sh
+defaults write com.google.Chrome ExternalProtocolDialogShowAlwaysOpenCheckbox -bool true
+```