add contributing section in README, partially resolves #2

Author: matthewhanson

README.md

@@ -80,3 +80,34 @@ Alibaba:
 - <https://www.alibabacloud.com/help/doc-detail/51374.htm>
 
 All timestamps MUST be formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6).
+
+## Contributing
+
+All contributions are subject to the
+[STAC Specification Code of Conduct](https://github.com/radiantearth/stac-spec/blob/master/CODE_OF_CONDUCT.md).
+For contributions, please follow the
+[STAC specification contributing guide](https://github.com/radiantearth/stac-spec/blob/master/CONTRIBUTING.md) Instructions
+for running tests are copied here for convenience.
+
+### Running tests
+
+The same checks that run as checks on PR's are part of the repository and can be run locally to verify that changes are valid. 
+To run tests locally, you'll need `npm`, which is a standard part of any [node.js installation](https://nodejs.org/en/download/).
+
+First you'll need to install everything with npm once. Just navigate to the root of this repository and on 
+your command line run:
+```bash
+npm install
+```
+
+Then to check markdown formatting and test the examples against the JSON schema, you can run:
+```bash
+npm test
+```
+
+This will spit out the same texts that you see online, and you can then go and fix your markdown or examples.
+
+If the tests reveal formatting problems with the examples, you can fix them with:
+```bash
+npm run format-examples
+```