Describe the rational versioning policy instead of SemVer

RubyGems and Bundler do not follow SemVer strictly, so claiming we do was
misleading. Spell out the actual policy: major may break existing code,
minor can change behavior while keeping existing functionality working,
and tiny is mostly bug fixes that may include small feature additions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hsbt

doc/NEW_FEATURES.md

@@ -9,7 +9,7 @@ To request substantial changes to Bundler or Bundler documentation, please start
 To add a new feature to Bundler:
 
 1. **Discuss your idea** - If the goal is already clear, link the existing [issue](https://github.com/rubygems/rubygems/issues) or [discussion](https://github.com/rubygems/rubygems/discussions) that covers it. Otherwise, post your idea in [GitHub Discussions](https://github.com/rubygems/rubygems/discussions) first so it can be talked through
-2. **Base on master branch** - We follow [SemVer](https://semver.org), so new features go in the master branch only, not old releases
+2. **Base on master branch** - Substantial new features go in the master branch only. Releases follow a rational versioning policy: major versions may break existing code, minor versions can change behavior or output while keeping existing functionality working, and tiny versions are mostly bug fixes that may include feature additions
 3. **Write tests** - Include at least one test covering your changes in your feature branch
 4. **Submit a pull request** - See [PULL_REQUESTS.md](PULL_REQUESTS.md) for detailed guidelines