Add Getting the Code section to Quick Start

[andrewloux]

What this PR does / why we need it:

• Want to keep dropping the barrier to developing on ingress-nginx, after having seen multiple colleagues attempt to figure it out.
• That the code has to be checked out under the k8s.io directory is not super intuitive, making it explicit here would be beneficial imho.

Special notes for your reviewer:
cc @aledbf

[codecov-io]

Codecov Report

Merging #2457 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2457      +/-   ##
==========================================
+ Coverage   41.61%   41.67%   +0.05%     
==========================================
  Files          74       74              
  Lines        5291     5291              
==========================================
+ Hits         2202     2205       +3     
+ Misses       2792     2790       -2     
+ Partials      297      296       -1

Impacted Files	Coverage Δ
cmd/nginx/main.go	23.44% <0%> (+2.06%)	⬆️

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[aledbf]

/lgtm
/approve

[aledbf]

@andrewlouis93 thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, andrewlouis93

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