Improve Compiling for iOS docs: Mention how to set up Xcode

contributing/development/compiling/compiling_for_ios.rst

@@ -16,6 +16,10 @@ Requirements
 - `Python 3.6+ <https://www.python.org/downloads/macos/>`_.
 - `SCons 3.1.2+ <https://scons.org/pages/download.html>`_ build system.
 - `Xcode <https://apps.apple.com/us/app/xcode/id497799835>`_.
+    - Launch Xcode once and install iOS support. If you have already launched
+      Xcode and need to install iOS support, go to *Xcode -> Settings... -> Platforms*.
+    - Go to *Xcode -> Settings... -> Locations -> Command Line Tools* and select
+      an installed version. Even if one is already selected, re-select it.
 
 If you are building the ``master`` branch: