add description for offline use

README.md

@@ -4,12 +4,35 @@ Online Zoom MS-50G/MS-70CDR multi stomp patch editor.
 This is a patch utility for ZOOM MS-50G/70CDR MultiStomp guitar pedal.  
 This is still a very tentative version for the confirmation of MIDI related function.
 
-Usage :
+## Usage
 * Connect PC/Mac to MS-50G/70CDR via USB
 * Launch this page. You should use latest Chrome
 * Press accept if 'MIDI device' dialog is displayed
 * Select MidiPort to 'ZOOM MS Series'
 
 [Zoom MS Utility Page](https://g200kg.github.io/zoom-ms-utility/)
 
+## To Use this tool in offline
+
+This tool will not work if you just open the local html file by Chrome because of file access security limitation. There are several ways to use this tool in an offline environment.
+
+1. Connect via the local server  
+  If you are already installing a full-fledged server such as Apache on your PC, place zoom-ms-utility in the document folder and connect via localhost. On Windows, you need to prepare your server program yourself. On Mac, Apache has already been installed, but configuration is necessary.
+    https://discussions.apple.com/docs/DOC-3083
+
+2. Connect via a simple local server.  
+  Simple local server is supported in script language environment like Python. They can be used as a local-server. For example Python:  
+ (1) install python 3.x.x and setup PATH  
+ (2) start server with command prompt > python -m http.server 8888  
+ (3) access by Chrome to http://localhost:8888/  
+
+3. Specify startup options for Chrome  
+  If `--allow-file-access-from-files` option is specified when launch Chrome, security restrictions on file access are canceled and zoom-ms-utility works  
+  For example on Windows,  
+  Make a shortcut of Chrome, like
+  `"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe"  --allow-file-access-from-files file:///C:/Users/xxxx/zoom-ms-utility/index.html`  
+  Or on Mac, use Automator shell script, like  
+  `Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --allow-file-access-from-files /Users/xxxx/zoom-ms-utility/index.html`
+
+## Info
 If you are interested in MS-50G/70CDR MIDI function, check `midimessage.md`