docs: add instructions for local testing

[mloiseleur]

What does it do ?

Provide an example on how to test locally External DNS.

Motivation

@TobyTheHutt found an interesting and fast way to test it (source). I would like to share it instead of keeping it hidden in PRs comment.

More

• Yes, this PR title follows Conventional Commits
• Yes, I updated end user documentation accordingly

[coveralls]

Pull Request Test Coverage Report for Build 20005100362

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 195 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.1%) to 78.684%

---

Files with Coverage Reduction	New Missed Lines	%
apis/externaldns/types.go	1	99.71%
plan.go	3	95.85%
source.go	3	83.33%
zz_generated.deepcopy.go	8	0.0%
istio_gateway.go	8	90.1%
txt.go	10	95.28%
endpoint.go	16	90.6%
service.go	20	93.33%
coredns/coredns.go	30	90.36%
gloo_proxy.go	47	74.18%

Totals
Change from base Build 18242454154:	0.1%
Covered Lines:	16009
Relevant Lines:	20346

---

💛 - Coveralls

[ivankatliarchuk]

/lgtm

[mloiseleur]

@vflaux @ivankatliarchuk Since there is now a nice coredns tutorial, I've updated this PR.
It now just links to it in the dev guide.

[ivankatliarchuk]

/lgtm
/approve

[k8s-ci-robot]

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