Add note about missing filters (#191)

Signed-off-by: Shriram Rajagopalan <shriram@us.ibm.com>

api/filter/README.md

@@ -0,0 +1,18 @@
+## NOTE
+
+The list of filters here is incomplete. There are no proto specifications for Fault filter, Redis filter, CORS filter, etc.
+These specifications will be added in the near future. In the interim, you can still supply plain JSON configuration objects
+for these missing filters by setting the `"deprecated_v1"` field to true in the filter's configuration. For example,
+
+```json
+{ 
+ "name": "envoy.rate_limit",
+  "config": { 
+    "deprecated_v1": true,
+     "value": { 
+       "domain": "some_domain",
+        "timeout_ms": 500 
+       }
+    }
+ }
+```