build: explain how to test Python 3.6 or 3.7

This helps contributors understand how they can test Python 3.6 or 3.7
on Travis CI.

Helps with issues like:
- #29246 (comment)

.travis.yml

@@ -6,9 +6,13 @@ x-ccache-setup-steps: &ccache-setup-steps
 
 os: linux
 language: cpp
+# Currently this file can only support one PYTHON_VERSION.
+# To experiment with Python 3, comment out Python 2.7 and uncomment one of the Python 3 versions.
 env:
   global:
     - PYTHON_VERSION="2.7.15"
+    # - PYTHON_VERSION="3.6.7"
+    # - PYTHON_VERSION="3.7.1"
 jobs:
   include:
     - stage: "Compile"