Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Supported Dart Versions
+Currently, `dpx` is written to support Dart 2.19 and Dart 3. We will eventually
+drop support for Dart 2, but until then, all dependency ranges need to take this
+into account and all CI checks (static analysis and tests) should pass on both
+major versions of Dart. Additionally, language features that require Dart 3+ are
+not yet used.
+
+## Running Locally
+
+- Install dependencies: `dart pub get`
+- Analysis: `dart analyze`
+- Tests: `dart test`
+- Running:
+  - `dart bin/dpx.dart`
+  - `dart pub global activate -spath .` to activate and `dart pub global run dpx`
+    to run (or just `dpx` if global Dart executables are added to your path).
+- Debugging:
+  - In VS Code, use the `DPX CLI` launch configuration. This will start `dpx` in
+    interactive mode and open a terminal where you can enter the args. This will
+    let you use the debugger and set breakpoints.