Add notes to publishing script

I read in the logs that the $DOCKER_USERNAME and $DOCKER_PASSWORD were 
printed out as:

+docker login -u [secure] -p [secure]

But, since this is a feature of travis rather than the script, let's 
avoid the risk of exposing secrets in logs by moving down the `set +x` 
call further.

cd/deploy.sh

@@ -1,9 +1,23 @@
 #!/bin/bash
 set -eu
+
+# Decrypt a private SSH key having its public key registered on GitHub. It will
+# be used to establish an identity with rights to push to the repo hosting our
+# Helm charts: https://github.com/jupyterhub/helm-chart
 openssl aes-256-cbc -K $encrypted_c6b45058ffe8_key -iv $encrypted_c6b45058ffe8_iv -in cd/id_rsa.enc -out cd/id_rsa -d
-set -x
 chmod 0400 cd/id_rsa
+
 docker login -u "${DOCKER_USERNAME}" -p "${DOCKER_PASSWORD}"
+
+# Activate logging of bash commands now that the sensitive stuff is done
+set -x
+
+# As chartpress utilizes git to push to our Helm chart repository, we configure
+# git ahead of time to utilize the identity we decrypted earlier.
 export GIT_SSH_COMMAND="ssh -i ${PWD}/cd/id_rsa"
+
 chartpress --commit-range "${TRAVIS_COMMIT_RANGE}" --push --publish-chart
+
+# Let us log the changes chartpress did, it should include replacements for
+# fields in values.yaml, such as what tag for various images we are using.
 git diff