client-go/examples/fake-client: add doc.go to fix go build warnings

[nikhita]

The publishing bot is currently broken (#56876 (comment)). This is due to the following error:

[24 Jun 18 15:08 UTC]: Running smoke tests for branch master
[24 Jun 18 15:08 UTC]: /bin/bash -xec "godep restore\ngo build ./...\ngo test $(go list ./... | grep -v /vendor/)\n"
	+ godep restore
	+ go build ./...
	go build k8s.io/client-go/examples/fake-client: no non-test Go files in /go-workspace/src/k8s.io/client-go/examples/fake-client
[24 Jun 18 15:09 UTC]: exit status 1
    	+ godep restore
    	+ go build ./...
    	go build k8s.io/client-go/examples/fake-client: no non-test Go files in /go-workspace/src/k8s.io/client-go/examples/fake-client

[24 Jun 18 15:09 UTC]: exit status 1```

The fakeclient package does not have any non-test go files. The test file was added in #65291 2 days ago.

This causes go build to give a warning: no non-test Go files in /go-workspace/src/k8s.io/client-go/examples/fake-client, which breaks the publishing bot. This PR adds a dummy doc.go file in the package to avoid this warning and fix the publishing bot.

Release note:

NONE

[nikhita]

/cc @sttts @mfojtik
since it fixes the publishing bot

[nikhita]

fyi @misterikkit

[nikhita]

/assign @liggitt
for approval since @sttts is out

[liggitt]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, nikhita

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[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[nikhita]

/retest

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 63773, 65411). If you want to cherry-pick this change to another branch, please follow the instructions here.

[k8s-ci-robot]

@nikhita: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-e2e-gce	764ae69	link	/test pull-kubernetes-e2e-gce

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[nikhita]

Just confirming here that the publishing bot works fine now. 👍