OS X uploader (#252)

* Source for OS X uploader

* Readme for OS X uploader

* Export image

* .gitignore for Xcode project

* Remove redundant data

* Update readme instructions

* List formatting

* Remove personal copyright notice added by Xcode

* Added release build and updated readme

* point to doc cdn

clients/macuploader/README.md

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+# micro:bit uploader for OS X
+
+![](Microbit Uploader/Assets.xcassets/AppIcon.appiconset/icon_256x256.png)
+
+This project is a clone of the [Windows
+uploader](https://codethemicrobit.com/uploader), but for OS X. Once launched,
+the app runs in your menu bar and will automatically deploy any HEX files to
+your `micro:bit`. Like the Windows version, it is compatible with any browser
+that can run [codethemicrobit.com](http://codethemicrobit.com).
+
+## Install the built version
+
+1. Download the latest `.zip` release from the `Release` directory
+2. Unzip it
+3. Drag `Microbit Uploader` to your Applications folder and launch it
+
+## Building
+
+To build the project you'll need a copy of OS X 10.11 or higher and Xcode 8 or
+higher (you may be able to build on earlier OSes or versions of Xcode, but this
+remains untested). Once you have a development environment set up, just build
+and run `Microbit Uploader.xcodeproj`.
+
+## Distributing
+
+1. Open the Xcode project
+2. Product > Archive
+3. Export:
+
+    ![Export](Graphics/export.png)
+
+4. You will then have the option of either signing or not-signing the
+   application:
+
+    a) If you have an Apple developer account, select 'Export a Developer
+       ID-signed Application'
+
+    b) If you don't have a developer ID, select 'Export as a macOS App'
+
+5. Zip the produced app and upload to CDN or equivalent