Add link to contributor guideline to quickstart docs

[khanhtc1202]

What this PR does:

as title

Why we need it:

Some newcomers want to set up dev environment for pipecd but stuck at quickstart guideline, so I think this note could help 👀

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.56%. Comparing base (abd6218) to head (31ca13f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5612   +/-   ##
=======================================
  Coverage   26.56%   26.56%           
=======================================
  Files         474      474           
  Lines       50535    50535           
=======================================
+ Hits        13423    13427    +4     
+ Misses      36048    36045    -3     
+ Partials     1064     1063    -1     

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