add WD06 link to editor's draft

Merge pull request #533 from AngeloKai/master

Author: Angelo Liao

index.bs

@@ -13,6 +13,7 @@ Status: ED
 Prepare for TR: true
 TR: https://www.w3.org/TR/webauthn/
 ED: https://w3c.github.io/webauthn/
+Previous Version: https://www.w3.org/TR/2017/WD-webauthn-20170811/
 Previous Version: https://www.w3.org/TR/2017/WD-webauthn-20170505/
 Previous Version: https://www.w3.org/TR/2017/WD-webauthn-20170216/
 Previous Version: https://www.w3.org/TR/2016/WD-webauthn-20161207/