add contributing guide, close #12

Author: bahmutov

CONTRIBUTING.md

@@ -0,0 +1,5 @@
+How to [code a plugin](https://github.com/netlify/build/blob/master/docs/creating-a-plugin.md)
+
+## Testing and releasing a new version
+
+Try using `beta` branch to release new pre-release versions of the plugin by following [the semantic release guide](https://github.com/semantic-release/semantic-release/blob/master/docs/recipes/pre-releases.md). You can fork and test out new versions published to NPM using the [netlify-plugin-cypress-example](https://github.com/cypress-io/netlify-plugin-cypress-example) repository. Hope the `master` branch merged into `beta` does not bring features and fixes *already released*. Thus I suggest using `beta` branch for new features.

README.md

@@ -141,4 +141,8 @@ Set environment variable `DEBUG=netlify-plugin-cypress` to see the debug logs. T
 
 ## License
 
-This project is licensed under the terms of the [MIT license](/LICENSE.md).
+This project is licensed under the terms of the [MIT license](LICENSE.md).
+
+## Contributing
+
+Read the [contributing guide](CONTRIBUTING.md)