Starting a readme for the cloud function. RE:natcap#1580

Author: phargogh

codesigning/gcp-cloudfunc/README.md

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+# GCP Cloud Function Source
+
+The files in this directory are used during deployment to the GCP Cloud Function.
+
+See the Makefile in the parent directory for the `gcloud` invokation to deploy the function.
+
+## NOTES:
+
+* After deploying the function for the first time, make sure the correct secret is defined as an environment variable.