doc: mention oneoflint in a README

Author: vyskocilm

README.md

@@ -3,6 +3,28 @@
 `sumlint` adds exhaustiveness checks for your interface "sum types" using a lightweight
 naming convention + `go vet`.
 
+`oneoflint` provides the same check for [gRPC oneof](https://protobuf.dev/programming-guides/proto3/#oneof).
+
+## Example
+
+```go
+// Non‑exhaustive: missing nonexhaustive.B, nonexhaustive.C.
+func bad(x SumFoo) {
+	switch x.(type) { // want `non-exhaustive type switch on SumFoo: missing cases for: nonexhaustive.B, nonexhaustive.C`
+	case A:
+	default:
+	}
+}
+
+// Non‑exhaustive: missing one_of.Msg_B
+func process(msg *Msg) {
+	switch msg.GetPayload().(type) { // want `non-exhaustive type switch on isMsg_Payload: missing cases for: one_of.Msg_B`
+	case *Msg_A:
+	default:
+	}
+}
+```
+
 Install:
 
 ```sh
@@ -75,6 +97,27 @@ type A struct{}
 func (A) sumFoo() {}
 ```
 
+## gRPC files
+
+It turned out code generated by `protoc` for Go uses almost same convention.
+
+```proto
+message Msg {
+  oneof payload {
+    A a = 1;
+    B b = 2;
+  }
+}
+```
+
+```go
+type isMsg_Payload interface {
+	isMsg_Payload()
+}
+```
+
+The `oneoflint` is the linter configured to check and report exactly this.
+
 ## 2. Exhaustive Type Switch Checking
 
 `sumlint` inspects type switches of all detected Sum* interfaces and reports two distinct problems.
@@ -120,8 +163,8 @@ $ cd tests; go vet -vettool=${HOME}/go/bin/sumlint .
 
 ## Conclusion
 
-`sumlint` lets you use exhaustively checked sum types in plain Go using a
-lightweight naming convention and `go vet`.
+`sumlint` and `oneoflint` lets you use exhaustively checked type switches in plain Go using a
+lightweight naming convention and `go vet` or other tool compatible with [golang.org/x/tools/go/analysis](https://pkg.go.dev/golang.org/x/tools/go/analysis).
 
 Happy vetting!