build: add comment to .travis.yml on how to test Py3

PR-URL: #29473
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

.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"