Add docker folder README

docker/README.md

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+# Docker containers for easy dependency bundling
+
+To facilitate downloading, building and running the required
+development tools, some of the tutorials in this repository use a
+container software called docker. While docker specifics are
+outside of this tutorial's scope it is worth mentioning that
+human readable docker configuration files named "Dockerfile"
+exist in the sub-directories of this folder.
+
+These files may be of interest to those who want details about
+what will happen when docker is invoked.