docs: add CONTRIBUTING.md

Author: larsgw

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+## Tests
+
+Run tests with the following:
+
+    npm test
+
+Or a specific test:
+
+    npm test -- --grep "TEST_NAME"
+
+### Adding tests
+
+Easily download metadata like so:
+
+    curl -LH 'accept: application/vnd.citationstyles.csl+json' https://doi.org/$DOI | jq '[.]' > test/$TEST_NAME.json
+
+Then, add a test in `test/suite.js`.