Add a documentation badge pointing to rubydoc.info

[elia]

Summary

Adds a badge to the main rubydoc.info url, it points to the latest rubygems released version.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [ ] I have written a thorough PR description.
• [ ] I have kept my commits small and atomic.
• [ ] I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[elia]

@kennyadsl yes, I explicitly avoided setting a version so it will always be the latest

[codecov[bot]]

Codecov Report

Merging #291 (8bae367) into master (a1b5866) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files          30       30           
  Lines         540      540           
=======================================
  Hits          538      538           
  Misses          2        2           

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