add development information

README.md

@@ -3,3 +3,46 @@
 Currency Converter for Firefox
 
 https://addons.mozilla.org/firefox/addon/scscurrencyconverter/
+
+## Development
+
+Requirements:
+
+- [Node.js](https://nodejs.org/)
+- [npm](https://github.com/npm/cli/releases/latest)
+
+### Install
+
+To install the project's dependencies:
+
+```sh
+npm install
+```
+
+### Start
+
+During development use:
+
+```sh
+npm start
+```
+
+This will build a develpment version of the scripts in the dist folder, and starts webpack in watch mode.
+
+In an other terminal/console:
+
+```sh
+npm run run:web-ext
+```
+
+This opens a clean Firefox with the addon installed. Every time a script is modified, the addon will be reloaded.
+
+### Build
+
+To build the addon, use:
+
+```sh
+npm run build
+```
+
+This will build a production version of the scripts in the dist folder, and packages them into a zip file in the build folder.