Add SUPPORT.md outlining our support policy.

Call out that v2 is generally available and will receive fixes, but that
we don't backport fixes to other minor versions. Generally lay out the
support policy we have been operating with thus far.

SUPPORT.md

@@ -0,0 +1,18 @@
+# Support Policy
+
+Version 2 of the Terraform Plugin SDK is considered **GA (generally
+available)** and has the following support policy:
+
+Critical bug fixes will be accepted and merged. New features and non-critical
+bug fixes may be accepted at maintainers’ discretion, but our priority is
+maintaining stability.
+
+We will not break the public interface exposed by the SDK in a minor or patch
+release unless a critical security issue or bug demands it, and only then as a
+last resort.
+
+We will continue testing version 2 of the SDK against new releases of Terraform
+to ensure compatibility.
+
+We will not be backporting bug fixes to prior minor releases. Only the latest
+minor release receives new patch versions.