Add subsection about Xcode license to Mac build instructions.

docs/mac_build_instructions.md

 # Mac Build Instructions
 
 [TOC]
 
 ## Prerequisites
 
 *   A Mac running 10.9+.
 *   [Xcode](https://developer.apple.com/xcode) 5+.
 *   [depot\_tools](http://dev.chromium.org/developers/how-tos/depottools).
 *   Install Xcode's "Command Line Tools" via Xcode menu -> Preferences ->
@@ skipping 233 matching lines @@
 be set in `/etc/sysctl.conf`:
 
     echo kern.maxvnodes=$((512*1024)) | sudo tee -a /etc/sysctl.conf
 
 Or edit the file directly.
 
 If your `git --version` reports 2.6 or higher, the following may also improve
 performance of `git status`:
 
     git update-index --untracked-cache
+
+## Xcode license agreement
+
+If you're getting the error
+
+```
+Agreeing to the Xcode/iOS license requires admin privileges, please re-run as root via sudo.
+```
+
+the Xcode license hasn't been accepted yet which (contrary to the message) any
+user can do by running:
+
+    xcodebuild -license
+
+Only accepting for all users of the machine requires root:
+
+    sudo xcodebuild -license