docs: add badge linking to pkg.go.dev

[binbin-li]

Description

What this PR does / why we need it:

Add a badge linking to pkg.go.dev: https://pkg.go.dev/github.com/deislabs/ratify

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Helm Chart Change (any edit/addition/update that is necessary for changes merged to the main branch)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -1.26% ⚠️

Comparison is base (0772c6c) 59.60% compared to head (018f55c) 58.34%.

❗ Current head 018f55c differs from pull request most recent head b7f67f5. Consider uploading reports for the commit b7f67f5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1056      +/-   ##
==========================================
- Coverage   59.60%   58.34%   -1.26%     
==========================================
  Files          75       93      +18     
  Lines        4510     5541    +1031     
==========================================
+ Hits         2688     3233     +545     
- Misses       1576     1992     +416     
- Partials      246      316      +70     

see 37 files with indirect coverage changes

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