.travis.yml: Explain why we re-checkout

deps/rabbitmq_stomp/.travis.yml

@@ -11,5 +11,11 @@ otp_release:
   - R16B03-1
   - "17.5"
   - "18.0"
+
+# The checkout made by Travis is a "detached HEAD". We switch back
+# to a tag or a branch. This pleases our git_rmq fetch method in
+# rabbitmq-components.mk and the proper tag/branch is selected in
+# dependencies too.
 before_script: (test "$TRAVIS_TAG" && git checkout "$TRAVIS_TAG") || (test "$TRAVIS_BRANCH" && git checkout "$TRAVIS_BRANCH")
+
 script: make tests