Adding documentation for default/oracle/oraclelinux7 variants which make it clear the OpenJDK official image is still released under the same GPLv2+CE license as before.

Signed-off-by: Avi Miller <[email protected]>

openjdk/variant-oracle.md

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+## `%%IMAGE%%:<version>` (from 12 onwards), `%%IMAGE%%:<version>-oracle` and `%%IMAGE%%:<version>-oraclelinux7`
+
+Starting with `openjdk:12` the default image as well as the `-oracle` and `-oraclelinux7` variants are based on the official [Oracle Linux 7 image](https://hub.docker.com/_/oraclelinux) which is provided under the GPLv2 as per the [Oracle Linux End User Agreement (EULA)](https://oss.oracle.com/ol7/EULA).
+
+The OpenJDK binaries in the default image as well as the `-oracle` and `-oraclelinux7` variants are built by Oracle and are sourced from the [OpenJDK community](https://openjdk.java.net/). These binaries are licensed under the [GPLv2 with the Classpath Exception](https://openjdk.java.net/legal/gplv2+ce.html).