First real version of README

README.md

+# Supported tags and respective Dockerfile links
+
+* `5.6`: [Dockerfile](5.6/Dockerfile)
+* `5.7`: [Dockerfile](5.7/Dockerfile)
+* `8`: [Dockerfile](8/Dockerfile)
+* `latest`: see `8`
+
+# Quick reference
+
+The images base on the official mysql images from docker. See [docker hub](https://hub.docker.com/_/mysql/)
+for the official details.
+
+The following changes were made:
+
+* Encoding of default database changed to UTF-8