Add information about the configuration to the README

README.md

 WBO is available in multiple languages. The translations are stored in [`server/translations.json`](./server/translations.json). 
 If you feel like contributing to this collaborative project, you can [translate WBO into your own language](https://github.com/lovasoa/whitebophir/wiki/How-to-translate-WBO-into-your-own-language).
 
+## Configuration
+
+When you start a WBO server, it loads its configuration from several environment variables.
+You can see a list of these variables in [`configuration.js`](./server/configuration.js).
+Some important environment variables are :
+ - `WBO_HISTORY_DIR` : configures the directory where the boards are saved. Defaults to `./server-data/`.
+ - `WBO_MAX_EMIT_COUNT` : the maximum number of messages that a client can send per unit of time. Increase this value if you want smoother drawings, at the expense of being susceptible to denial of service attacks if your server does not have enough processing power. 
+
 ## Troubleshooting
 
 If you experience an issue or want to propose a new feature in WBO, please [open a github issue](https://github.com/lovasoa/whitebophir/issues/new).