docs: add contributing guide

Author: alysnnix

CONTRIBUTING.md

@@ -0,0 +1,54 @@
+# Contributing to devproxy
+
+Thanks for your interest in contributing!
+
+## Reporting bugs
+
+Open an issue using the **Bug report** template. Include:
+
+- What happened and what you expected
+- Steps to reproduce
+- Output of `devproxy doctor` and relevant daemon logs
+- Your OS, Docker version, and devproxy version
+
+## Suggesting features
+
+Open an issue using the **Feature request** template. Describe the problem you're trying to solve and your proposed solution.
+
+## Submitting PRs
+
+1. Fork the repo and create a branch from `main`
+2. Make your changes
+3. Run tests: `go test ./internal/... ./cmd/... -v`
+4. Run vet: `go vet ./...`
+5. Open a PR against `main`
+
+### Requirements
+
+- All tests must pass
+- New functionality should include tests
+- Keep changes focused — one concern per PR
+
+### Building
+
+```bash
+# With Nix
+nix build
+
+# With Go
+go build -o devproxy ./cmd/devproxy
+```
+
+### Running tests
+
+```bash
+# Unit tests
+go test ./internal/... ./cmd/...
+
+# Integration tests (requires Docker + root)
+sudo go test -tags=integration ./integration/ -v
+```
+
+## Code of conduct
+
+Be respectful. We're all here to build something useful.