update readme

Readme.md

@@ -93,6 +93,36 @@ List of possible values of `<task-name>`:
 
 [1] Running this task on macOS or Linux requires Wine, since it's needed to set the icon for the Windows app (build for specific platform to avoid errors).
 
+#### Setting up and building on a Mac
+
+- Install GitHub desktop application from https://desktop.github.com and open the GitHub Desktop application.
+- At https://github.com/betaflight/betaflight-configurator, select Clone or Download > Open in Desktop
+
+(The GitHub Desktop application should come to the front and create a repository (not necessarily where you want it).  The blackbox-log-viewer repository (folder) should appear under the list of local repositories.  You can find your local repository location on your mac using the 'Locate in Finder' command GitHub Desktop  It can be moved somewhere more else, but you'll then need to tell Github where you're moved it to.)
+
+Open Terminal.app and install or update homebrew:
+```
+/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
+```
+install node 8.x and npm, if already installed, agree to update them
+```
+brew install node@8 npm
+```
+Change Terminal's working directory wherever you put blackbox-log-viewer folder; easiest way is to type 'cd ' in Terminal then drag the blackbox-log-viewer folder from the Finder to the terminal window.  Or use a terminal command like 
+```
+cd ~/mydirectorypath/blackbox-log-viewer
+```
+
+install dependencies into that folder (ignoring many confusing messages) with:
+```
+npm i
+```
+
+finally build the DMG itself, which will end up in blackbox-log-viewer/release/, with:
+```
+npx gulp release --osx64
+```
+
 #### Build or release app for one specific platform
 To build or release only for one specific platform you can append the plaform after the `task-name`.
 If no platform is provided, only for the platform you are builing from will be build.