cleanup readme, add explanation for MB_PYTHON_OSX_VER

README.rst

@@ -57,10 +57,17 @@ functions and variables in earlier scripts:
 
 See ``travis_osx_steps.sh`` to review source order.
 
-The macOS build / test and phase are on the macOS VM started by Travis CI.
+The macOS build / test phases run on the macOS VM started by Travis CI,
 Therefore any environment variable defined in the ``.travis.yml`` or bash
 shell scripts listed above are available for your build and test.
 
+Note that macOS builds may be targetted either at macOS 10.6 and above
+(dual arch 32/64b) or macOS 10.9 and above (64b only), depending on the
+value of the environment variable `MB_PYTHON_OSX_VER`. These depend on the
+corresponding build of python from https://www.python.org/downloads/mac-osx/.
+Versions of python older than 3.6.5 and 2.7.15 don't have 10.9 / 64-bit only
+installers available.
+
 The ``build_wheel`` function builds the wheel, and the ``install_run``
 function installs the wheel and tests it.  Look in ``common_utils.sh`` for
 default definitions of these functions.  See below for more details.
@@ -258,18 +265,7 @@ To use these scripts
         - os: osx
           language: generic
           env:
-            - MB_PYTHON_VERSION=3.6
-        - os: osx
-          language: generic
-          env:
-            - MB_PYTHON_VERSION=3.6
-            - MB_PYTHON_OSX_VER=10.9
-        # Default OSX (xcode image) is 10.13 (xcode 9.4.1) as of 2018-08-03
-        # See: https://docs.travis-ci.com/user/reference/osx/
-        - os: osx
-          osx_image: xcode10.1
-          env:
-            - PYTHON_VERSION=3.7
+            - MB_PYTHON_VERSION=3.7
             - MB_PYTHON_OSX_VER=10.9
         - os: osx
           language: generic
@@ -315,6 +311,7 @@ To use these scripts
     install:
         - build_index_wheel $PROJECT_SPEC
 
+
 * Next create a ``config.sh`` for your project, that fills in any steps you
   need to do before building the wheel (such as building required libraries).
   You also need this file to specify how to run your tests::