Updating contributing guidance and readme

Author: mccaffers

.github/workflows/auto-close-external-prs.yml

@@ -20,7 +20,7 @@ jobs:
               owner: context.repo.owner,
               repo: context.repo.repo,
               issue_number: pr,
-              body: "Thanks for the interest! This repository isn't currently accepting external contributions as I am actively experimenting with different approaches and want to avoid merge conflicts. Therefore, this PR is being closed automatically by Github Bot."
+              body: "Thanks for the interest! This repository isn't currently accepting external contributions as I am actively experimenting with different approaches and want to avoid merge conflicts. Therefore, this PR is being closed automatically by Github Bot. See [CONTRIBUTING.md](https://github.com/mccaffers/backtesting-engine-cpp/blob/main/CONTRIBUTING.md) for how to fork or file an issue instead."
             });
             await github.rest.pulls.update({
               owner: context.repo.owner,

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contributing
+
+Thanks for taking a look! This is an active solo experiment, and I'm grateful for the interest.
+
+## I'm not accepting pull requests right now
+
+This repository isn't currently accepting external contributions. I am actively experimenting with different approaches and want to avoid merge conflicts, so any PR opened from a non-owner account will be commented on and closed automatically by a GitHub bot.
+
+Please don't take it personally — the policy is the same for everyone, and it isn't a judgement on the change.
+
+## Please fork freely
+
+The project is [MIT-licensed](LICENSE.MD), so you're very welcome to fork it and take the code in your own direction. If your fork sparks a discussion or you want to share what you've built on top of it, feel free to open an issue (see below).
+
+## Use GitHub Issues for bugs, questions, and ideas
+
+Bugs, questions, and ideas are all welcome on the [Issues tab](https://github.com/mccaffers/backtesting-engine-cpp/issues). That's the supported way to flag something — much more useful to me than a PR I'd have to close.
+
+When filing an issue, a short description of what you saw, what you expected, and (for bugs) how to reproduce it goes a long way.
+
+## A note on `@claude` mentions
+
+This repo uses the [Claude Code GitHub Action](https://github.com/anthropics/claude-code-action) on `@claude` mentions, but it's restricted to the repository owner — `@claude` comments from other accounts won't trigger a run. This keeps paid API usage under control.

README.md

@@ -82,5 +82,9 @@ Xcode - Library Path
 
 `bash ./scripts/test.sh`
 
+### Contributing
+
+This is an active solo experiment, so I'm not accepting pull requests right now — but please fork freely and use [GitHub Issues](https://github.com/mccaffers/backtesting-engine-cpp/issues) for bugs, questions, and ideas. See [CONTRIBUTING.md](CONTRIBUTING.md) for details.
+
 ### License
 [MIT](https://choosealicense.com/licenses/mit/)