Add some docs

docs/configuration.md

@@ -0,0 +1,40 @@
+# Configuration
+
+First of all, I'll give an overview about the configuration options. See the 
+[config.js](https://github.com/psi-4ward/psitransfer/blob/master/config.js#L5) for 
+possible values. I do **not** recommend changing this file directly, better use one
+of the following options.
+
+## Config file: NODE_ENV related
+
+PsiTransfer searches for an config file with the name `config.<NODE_ENV>.js` in the
+root folder where `<NODE_ENV>` stands for the value of the environment parameter `NODE_ENV`.
+If you start PsiTransfer using `npm start` it's `production` so you can create a
+`config.production.js` with your settings. For example take a look at 
+[config.dev.js](https://github.com/psi-4ward/psitransfer/blob/master/config.dev.js).
+This file is used when starting the application with `npm run dev`.
+
+You are completely free to introduce own configs like `config.custom.js` and start
+the app with `NODE_ENV=custom node app.js`.
+
+## Environment variables
+
+Some Linux distributions have `/etc/default/<daemon>` or `/etc/sysconfig/<daemon>`
+files with environment configurations. Moreover, it's common to 
+configure the behaviour of Docker containers using environment parameters.
+
+PsiTransfer supports overwriting every config value by environment parameters prefixed
+with `PSITRANSFER_`.
+
+```bash
+export NODE_ENV=dev
+export PSITRANSFER_RETENTIONS='{"one-time":"one time","3600":"1 Hour"}'
+export PSITRANSFER_PORT=8080
+node app.js
+```
+
+* The above example sets the `NODE_ENV` to `dev`.  
+  If `config.dev.js` exists, it is loaded and overwrites the corresponding values from `config.js`.
+* Then it will overwrite `retentions` and `port` with the values of the environment parameters.
+
+> Environment parameters always have the highest priority.