Add step to install Xcode command line tools

Author: jonreid

README.md

@@ -11,11 +11,12 @@ modifications.
 2. Place the XcodeCoverage folder in the same folder as your Xcode project.
 3. [Dowload lcov-1.10](http://downloads.sourceforge.net/ltp/lcov-1.10.tar.gz).
 Place the lcov-1.10 folder inside the XcodeCoverage folder.
-4. In your Xcode project, enable these two build settings at the project level
+4. Get Xcode's coverage instrumentation by going to Xcode Preferences, into Downloads, and installing Command Line Tools.
+5. In your Xcode project, enable these two build settings at the project level
 for your Debug configuration only:
   * Instrument Program Flow
   * Generate Test Coverage Files
-5. In your main target, add a Run Script build phase to execute
+6. In your main target, add a Run Script build phase to execute
 ``XcodeCoverage/exportenv.sh``