add note about polyfills

Co-authored-by: Kristján Oddsson <[email protected]>

Author: keithamus

README.md

@@ -34,6 +34,18 @@ npm install
 npm test
 ```
 
+## Contributing
+
+### Adding polyfills
+
+Please do not add any polyfills for ECMA features that are Stage 3 or below. We _only_ wish to polyfill features from ECMAScript that are Stage 4 (about to be included in a new years specification) or already specified.
+
+### Removing polyfills
+
+When our supported browsers (see github/github for details) all support a feature without polyfills, we can remove the polyfill. This code is designed to be kept lightweight, we do not want to ship dozens of kb of polyfills.
+
+As a polyfill is removed, it may be worth adding feature detection to the `baseSupport` const, to ensure that our baseline moves with our browser support matrix.
+
 ## License
 
 Distributed under the MIT license. See LICENSE for details.