Starting v3 release for plugin developers.

[doc]: https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher

Per this Go modules [doc] a repo or branch that's
already tagged v2 or higher should increment the major
version (e.g. go to v3) when releasing their first Go
module-based packages.

At the moment, the kustomize repo has these top level
packages in the sigs.k8s.io/kustomize module:

 - `cmd` - holds main program for kustomize

	 Conceivably someone can depend on this
	 package for integration tests.

 - `internal` - intentionally unreleased subpackages

 - `k8sdeps` - an adapter wrapping k8s dependencies

	 This exists only for use in pre-Go-modules kustomize-into-kubectl
	 integration and won't live much longer (as everything involved is
	 switching to Go modules).

 - `pkg` - kustomize packages for export

	 This should shrink in later versions, since
	 the surface area is too large, containing
	 sub-packages that should be in 'internal'.

 - `plugin` - holds main programs for plugins

This PR changes the top level go.mod file from

```
module sigs.k8s.io/kustomize
```

to

```
module sigs.k8s.io/kustomize/v3
```

and adjusts all import statements to
reflect the change.

examples/README.md

@@ -9,7 +9,7 @@ tests, and should work with HEAD
 
 <!-- @installkustomize @test -->
 ```
-go get sigs.k8s.io/kustomize/cmd/kustomize
+go get sigs.k8s.io/kustomize/v3/cmd/kustomize
 ```
 
 Basic Usage

examples/zh/README.md

@@ -7,7 +7,7 @@
 这些示例通过了 [pre-commit](../../travis/pre-commit.sh) 测试，并且应该与 HEAD 一起使用。
 
 ```
-go get sigs.k8s.io/kustomize
+go get sigs.k8s.io/kustomize/v3/cmd/kustomize
 ```
 
 基本用法