Improve docs for piped-plugin-sdk-go

[Warashi]

What this PR does:

• Add a note on where to create issues
• Improve wordings

Why we need it:

We manage the SDK codebase in this repository, so it's reasonable that we manage issues in this repository too.

Which issue(s) this PR fixes:

Part of #5530

Does this PR introduce a user-facing change?: No

• 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 28.50%. Comparing base (f39124f) to head (f3acfa9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5957   +/-   ##
=======================================
  Coverage   28.50%   28.50%           
=======================================
  Files         520      520           
  Lines       56434    56434           
=======================================
  Hits        16084    16084           
  Misses      39083    39083           
  Partials     1267     1267           

Flag	Coverage Δ
.	23.80% <ø> (ø)
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	48.86% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[khanhtc1202]

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[ffjlabo]

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