Add comment for Travis branch/tag build spec

Author: altendky

.travis.yml

@@ -2,6 +2,8 @@ language: python
 os: linux
 branches:
   only:
+    # Build tags and branches named master or starting with
+    # 'v' followed by a digit, such as for version tags.
     - master
     - /^v\d.*$/
 cache: pip
@@ -44,7 +46,7 @@ install:
         export PATH=$(echo $HOME/pypy3*/bin):$PATH
         export TOXPYTHON=$(echo $HOME/pypy3*/bin/pypy3)
     fi
-    set +x
+    set +x starting
   - |
     set -ex
     if [[ $TOXENV == 'check' ]]; then