Update README with information about visibility errors related to swift-proto.

Author: timburks

README.md

@@ -58,6 +58,20 @@ testing with the following versions:
 - Swift 3.1 (swiftlang-802.0.53 clang-802.0.42)
 - swift-protobuf 0.9.901 
 
+Also, please note that recent changes to swift-proto cause generated structs
+to have the "internal" access level. To allow them to be used in the
+public interfaces generated by the grpc-swift plugin, please generate 
+your proto structs with the following protoc options:
+
+	protoc <your proto files> \
+    	--swift_out=. \
+    	--swift_opt=Visibility=Public \
+    	--plugin=<path to protoc-gen-swift>`
+
+More details on the `Visibility` option are [here](https://github.com/apple/swift-protobuf/blob/master/Documentation/PLUGIN.md#generation-option-visibility---visibility-of-generated-types).
+Due to limitations in `protoc`, the `--plugin` option must be used to 
+specify the path to `protoc-gen-swift` whever the `--swift_opt` flag is used.
+
 ## License
 
 grpc-swift is released under the same license as