Document use of Xcode 7.3.1 with 10.10 SDK for release builds

docs/development/build-instructions-osx.md

@@ -5,7 +5,7 @@ Follow the guidelines below for building Electron on macOS.
 ## Prerequisites
 
 * macOS >= 10.8
-* [Xcode](https://developer.apple.com/technologies/tools/) >= 5.1
+* [Xcode](https://developer.apple.com/technologies/tools/) >= 7.3.1
 * [node.js](http://nodejs.org) (external)
 
 If you are using the Python downloaded by Homebrew, you also need to install
@@ -13,6 +13,26 @@ the following Python modules:
 
 * [pyobjc](https://pythonhosted.org/pyobjc/install.html)
 
+## macOS SDK
+
+If you're simply developing Electron and don't plan to redistribute your
+custom Electron build, you may skip this section.
+
+For certain features (e.g. pinch-zoom) to work properly, you must target the
+macOS 10.10 SDK.
+
+Official Electron builds are built with Xcode 7.3.1, which does not contain
+the 10.10 SDK by default. To obtain it, first download and mount the
+[Xcode 6.4](http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_6.4/Xcode_6.4.dmg)
+DMG.
+
+Then, assuming that the Xcode 6.4 DMG has been mounted at `/Volumes/Xcode` and
+that your Xcode 7.3.1 install is at `/Applications/Xcode.app`, run:
+
+```bash
+cp -r /Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
+```
+
 ## Getting the Code
 
 ```bash