prep for 1.6.0 release

[untra]

🔤 Polyglot PR

• remove github-pages gem (not used as part of publish process)
• adding a CONTRIBUTING.md file
• prep for a 1.6.0 release with support from @aturret
• added a test for checking frozen string output can be relativized

Type of change

• Docs update (changes to the readme or a site page, no code changes)
• Ops wrangling (automation or test improvements)
• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Sweet release (needs a lot of work and effort)
• Something else (explain please)

Upcoming release of fix for nil output from attempts to run gsub! on frozen strings

Checklists

• If modifying code, at least one test has been added to the suite