Add CONTRIBUTING.md (peter-murray#7)

CONTRIBUTING.md

@@ -0,0 +1,33 @@
+# Contributing
+
+Thanks for your interest in contributing!
+
+If your idea is a substantial change to how the action works, please [raise an issue](https://github.com/peter-murray/workflow-application-token-action/issues/new) to discuss before raising a pull request.
+
+## Running the tests
+
+The first thing to do is ensure that your tests run successfully:
+
+- Create an application [as described in the README](https://github.com/peter-murray/workflow-application-token-action#creating-a-github-application)
+- Create a file in your home folder named `.github_application` with the following contents (replacing the values with your own application):
+
+```json
+{
+  "test": {
+    "applicationId": 123456,
+    "privateKey": "-----BEGIN RSA PRIVATE KEY-----\nk3y_g03s_her3\n-----END RSA PRIVATE KEY-----\n",
+    "repo": {
+      "owner": "your-org-name",
+      "repo": "your-repo-name"
+    },
+    "org": "your-org-name"
+  }
+}
+```
+
+> If you need to get your private key on a single line, run `awk -v ORS='\\n' '1' /path/to/key.pem`
+
+- Run `npm install`
+- Run `npm test`
+
+You should see something similar to `15 passing (5s)` - if you see any failures, open an issue to resolve that **before** making any code changes.