Add contributing guidelines (#53)

Signed-off-by: Gray Campbell <12163070+graycampbell@users.noreply.github.com>

Author: graycampbell

CONTRIBUTING.md

@@ -0,0 +1,65 @@
+# Contributing
+
+> [!IMPORTANT]
+> PRs are merged at the discretion of the project's maintainers. If you would like to contribute code to this project,
+> please first [open an issue](https://github.com/fetch-rewards/SwiftSyntaxSugar/issues/new) with a detailed description
+> of your proposed changes. This allows us to discuss implementation, alternatives, etc. and avoid wasting time dealing
+> with the inefficient back and forth that can arise when PRs are created without prior discussion.
+>
+> After you've created an issue, please read through the guidelines in this document carefully before implementing any
+> changes.
+>
+> By opening an issue or contributing code to this project, you agree to follow our
+> [Code of Conduct](https://github.com/fetch-rewards/SwiftSyntaxSugar/blob/main/CODE_OF_CONDUCT.md).
+> 
+> Thank you for helping us make this project the best it can be!
+
+- [Signed Commits Required](#signed-commits-required)
+- [Commit Messages & PR Titles](#commit-messages--pr-titles)
+
+## Signed Commits Required
+
+All contributions to this project must use **signed commits**. This is an important part of our commitment to security, 
+authenticity, and trust in the software we maintain. Signed commits prove that a commit actually came from you, not just 
+someone who knows your name and email. Without signed commits, it’s possible for malicious actors to impersonate contributors 
+and inject malicious code into the project.
+
+> [!IMPORTANT]
+> Unsigned commits will be automatically rejected by our CI. If you forget to sign a commit, you can amend and re-push.
+
+To learn more about commit signature verification and to make sure you're using signed commits, please read this
+[guide](https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification).
+
+Thank you for helping us make the open-source community safer!
+
+## Commit Messages & PR Titles
+
+Descriptive, well-formatted commit messages and PR titles help create a consistent experience for maintainers and
+contributors.
+
+**All commits and PR titles should follow these guidelines:**
+
+1. The subject is written using sentence case, not title case, and acronyms are written in all caps:
+   ```
+   ✅ Make public API changes
+   ❌ Make public api changes
+   ❌ Make Public API Changes
+   ❌ MAKE PUBLIC API CHANGES
+   ```
+1. The subject is written in the imperative:
+   ```
+   ✅ Add new file
+   ❌ Adds new file
+   ❌ Added new file
+   ❌ Adding new file
+   ```
+1. The subject does not end with a period or include unnecessary punctuation:
+   ```
+   ✅ Refactor networking layer
+   ❌ Refactor networking layer.
+   ```
+1. The subject is ideally 50 characters or less (otherwise, 72 characters or less).
+1. The subject is separated from the body with a blank line (critical unless the body is ommitted entirely).
+1. The subject and body are free of whitepsace errors and typos.
+1. The body uses proper punctuation and capitalization.
+1. The body has a line length of 72 characters or less.