docs: add developer docs

CONTRIBUTING.md

@@ -37,6 +37,10 @@ There are git hooks set up with this project that are automatically installed
 when you install dependencies. They're really handy, and will take care of linting
 and formatting for you.
 
+## Thinking of a new feature?
+
+Make sure you read our [features guidelines](/docs/features.md).
+
 ## Help needed
 
 Please checkout the [the open issues][issues]

docs/features.md

@@ -0,0 +1,26 @@
+## Contributing with a new feature
+
+If you are thinking about adding a new feature to [Testing Playground](https://testing-playground.com/) and wondering where to start, you are in the right place.
+
+First, make sure that doesn't exist yet as [feature request](https://github.com/testing-library/testing-playground/issues?q=is%3Aissue+is%3Aopen+label%3Afeature). If it doesn't, go ahead and [create it](https://github.com/testing-library/testing-playground/issues/new?labels=feature&template=2.feature.md).
+
+We will advise you to first gather some feedback from the community and then proceed with coding.
+
+The project is divided on two applications:
+
+- [The website](https://testing-playground.com/) (in `src/`)
+- [The Extension](https://chrome.google.com/webstore/detail/testing-playground/hejbmebodbijjdhflfknehhcgaklhano?hl=en) (in `devtools/`)
+
+Have a look at the code to get the big picture, and follow the patterns we are currently using (naming conventions, folder structure...etc.).
+
+This is how our component hierarchy looks like right now:
+
+![Component hierarchy](https://user-images.githubusercontent.com/1196524/85944695-17b16e80-b939-11ea-922b-ab00245a364a.png)
+
+## You should also know...
+
+- We use [Taildwind](https://tailwindcss.com/) for CSS.
+- We use [ReachUI](https://reach.tech/) for interactive UI elements
+- We use [Octicons](https://primer.style/octicons/) icons
+- We use [undraw](https://undraw.co/illustrations) for illustrations
+  - With color `#EDF2F7` and `.5` opacity.