Documentation, CHANGELOG: use new go.etcd.io/etcd/v3 pkg

Use the new package path in the docs and announce it in the CHANGELOG

CHANGELOG-3.5.md

@@ -21,6 +21,7 @@ See [code changes](https://github.com/etcd-io/etcd/compare/v3.4.0...v3.5.0) and
 
 ### Breaking Changes
 
+- `go.etcd.io/etcd` Go packages have moved to `go.etcd.io/etcd/v3` to follow the [Go modules](https://github.com/golang/go/wiki/Modules) conventions
 - Changed behavior of clienv3 API [MemberList](https://github.com/etcd-io/etcd/pull/11639).
   - Previously, it is directly served with server's local data, which could be stale.
   - Now, it is served with linearizable guarantee. If the server is disconnected from quorum, `MemberList` call will fail.

Documentation/dev-guide/grpc_naming.md

@@ -10,8 +10,8 @@ The etcd client provides a gRPC resolver for resolving gRPC endpoints with an et
 
 ```go
 import (
-	"go.etcd.io/etcd/clientv3"
-	etcdnaming "go.etcd.io/etcd/clientv3/naming"
+	"go.etcd.io/etcd/v3/clientv3"
+	etcdnaming "go.etcd.io/etcd/v3/clientv3/naming"
 
 	"google.golang.org/grpc"
 )

Documentation/dl-build.md

@@ -28,8 +28,8 @@ To build a vendored `etcd` from the `master` branch via `go get`:
 # GOPATH should be set
 $ echo $GOPATH
 /Users/example/go
-$ go get -v go.etcd.io/etcd
-$ go get -v go.etcd.io/etcd/etcdctl
+$ go get -v go.etcd.io/etcd/v3
+$ go get -v go.etcd.io/etcd/v3/etcdctl
 ```
 
 ## Test the installation

client/README.md

@@ -9,7 +9,7 @@ For full compatibility, it is recommended to vendor builds using etcd's vendored
 ## Install
 
 ```bash
-go get go.etcd.io/etcd/client
+go get go.etcd.io/etcd/v3/client
 ```
 
 ## Usage
@@ -22,7 +22,7 @@ import (
 	"time"
 	"context"
 
-	"go.etcd.io/etcd/client"
+	"go.etcd.io/etcd/v3/client"
 )
 
 func main() {

pkg/adt/README.md

@@ -16,7 +16,7 @@ For example,
 import (
     "fmt"
 
-    "go.etcd.io/etcd/pkg/adt"
+    "go.etcd.io/etcd/v3/pkg/adt"
 )
 
 func main() {