chore(AIP-161): Link AIP-157 from AIP-161 (#1503)

Author: icmdaf

aip/general/0161.md

@@ -28,7 +28,8 @@ common on Update requests (AIP-134).
 Field masks **must** always be relative to the resource:
 
 **Warning:** Read masks as a single field on the request message, for
-example: `google.protobuf.FieldMask read_mask` are **DEPRECATED**.
+example: `google.protobuf.FieldMask read_mask` are **DEPRECATED**. Instead, see
+AIP-157.
 
 ```proto
 message UpdateBookRequest {