Recommend checked-in binstubs

[OskarEichler]

Summary

• recommend generating application binstubs locally and committing them
• explain why shared deploy-time binstubs can retain reaped release paths
• keep the existing runtime settings for backwards compatibility while removing the unsafe recommendation

Closes #111.

Verification

• reviewed the current Bundler 4 binstub overwrite behavior (--force is required to replace an existing binstub)
• checked the README diff with git diff --check

Documentation only; no runtime behavior changes.