chore: add contributing section to readme (#1858)

Signed-off-by: Diana Lau <[email protected]>

Author: dhmlau

README.md

@@ -33,3 +33,18 @@ npm install loopback-connector-mongodb // in this case, the mongodb connector
 See the [LoopBack documentation](http://loopback.io/doc/en/lb3/index.html).
 
 For information on data source connectors, see [Connecting models to data sources](https://loopback.io/doc/en/lb3/Connecting-models-to-data-sources.html).
+
+
+## Contributing 
+
+This project uses [DCO](https://developercertificate.org/). Be sure to sign off
+your commits using the `-s` flag or adding `Signed-off-By: Name<Email>` in the
+commit message.
+
+**Example**
+
+```
+git commit -s -m "feat: my commit message"
+```
+
+Also see the [Contributing to LoopBack](https://loopback.io/doc/en/contrib/code-contrib.html) to get you started.