Added Publication Status

Agreement reached in Community Group:
https://www.w3.org/community/solid/wiki/Meetings#20200109_1000CET

Author: csarven

README.md

@@ -4,6 +4,8 @@
 
 **Current version:** `v.0.7.0` (see [CHANGELOG.md](CHANGELOG.md))
 
+**Publication status**: Unofficial Draft
+
 **This document contains an informal description of implementation guidelines for Solid servers and clients.
 A normative specification is in the making at https://github.com/solid/specification/.
 For the time being, the present document contains the best approximation of expected server and client behavior.**