Add development.md

DEVELOPMENT.md

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+## Setup
+
+To run the app you will need [yarn](https://classic.yarnpkg.com/en/docs/install) installed on your machine. Then run the following command to install the dependencies:
+
+```
+yarn install
+```
+
+The next step is creating a `.env` file in the project's root. Its content should mirror the `.env.example`.
+
+Now you should be able to run the app with:
+
+```
+yarn start
+```
+
+## Preview
+
+You can also run a preview of the built app, which starts a server in the `dist` folder:
+
+```
+yarn build
+yarn preview
+```