Add instructions for deploying DomainMapping feature to DEVELOPMENT.md

[julz]

Part of #9713.

Documents how to deploy DomainMapping in DEVELOPMENT.md.

[markusthoemmes]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julz, markusthoemmes

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[codecov]

Codecov Report

Merging #9964 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9964      +/-   ##
==========================================
+ Coverage   87.81%   87.83%   +0.02%     
==========================================
  Files         183      183              
  Lines        8631     8631              
==========================================
+ Hits         7579     7581       +2     
+ Misses        802      800       -2     
  Partials      250      250              

Impacted Files	Coverage Δ
pkg/autoscaler/statforwarder/forwarder.go	81.18% <0.00%> (-1.08%)	⬇️
pkg/activator/net/revision_backends.go	91.40% <0.00%> (+0.90%)	⬆️
pkg/reconciler/revision/reconcile_resources.go	83.13% <0.00%> (+2.40%)	⬆️

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