README.md: mention protopath and protorange packages

Change-Id: I33045b4356642d433d88112cf0c6e6210dace58e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/331229
Trust: Joe Tsai <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: dsnet

README.md

@@ -68,6 +68,11 @@ Summary of the packages provided by this module:
     Package `protodesc` provides functionality for converting
     `descriptorpb.FileDescriptorProto` messages to/from the reflective
     `protoreflect.FileDescriptor`.
+*   [`reflect/protopath`](https://pkg.go.dev/google.golang.org/protobuf/reflect/protopath):
+    Package `protopath` provides a representation of a sequence of
+    protobuf reflection operations on a message.
+*   [`reflect/protorange`](https://pkg.go.dev/google.golang.org/protobuf/reflect/protorange):
+    Package `protorange` provides functionality to traverse a protobuf message.
 *   [`testing/protocmp`](https://pkg.go.dev/google.golang.org/protobuf/testing/protocmp):
     Package `protocmp` provides protobuf specific options for the `cmp` package.
 *   [`testing/protopack`](https://pkg.go.dev/google.golang.org/protobuf/testing/protopack):