Add security notice as advised

Author: CoMPaTech

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+As spare-time/free-time developers we support this python module as best
+as we can. We will support released versions as old as the module version
+included in the `manifest.json` of the current and previous releases of
+[Home Assistant Core](https://github.com/home-assistant/core) and/or
+[`plugwise-beta`](https://github.com/plugwise/plugwise-beta)
+(whichever used release is the oldest).
+
+## Reporting a Vulnerability
+
+Create an issue in this repository using the `bug` template. We're
+frequent visitors of our own repositories and will try to provide a
+solution as quick as we can. We are available on discourse using our
+github aliases for quicker response, but always file an issue so other
+users are also informed.
+
+(Security) fixes will always be pre-released and released through
+[`plugwise-beta`](https://github.com/plugwise/plugwise-beta) for
+testing. A PR to HA Core will follow either in parallel or as soon as
+possible.
+
+Thanks in advance for helping us keeping the community safe.