Add CONTRIBUTING.md

Author: doesdev

CONTRIBUTING.md

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+### Contributing code
+- fork
+- create (feature / fix) branch in your fork
+- add tests
+- use `js standard` code style
+- open pull request
+
+### Notes
+Tests are written against several Rollup versions. As such you must use Yarn to manage / install deps. 
+Also when adding tests be aware that they are run in a loop of each of the tested Rollup versions. 
+Of course there have been some API changes in Rollup over time so you may need to make adjustments 
+when testing agsinst one version versus another. As an example you'll notice the tree-shaking tests 
+are limited to versions 0.60.x and up.