Explains config (#94)

[ci skip]

Author: austinpray

README.md

@@ -58,13 +58,41 @@ npm i --progress=false
 
 ### Project Structure
 
-Source lives in `src/`. Compiled output goes in `dist/`.
+Source lives in `src/`. Compiled output goes in `dist/`. Configuration goes in
+`config/`.
 
 #### Source Files
 
 - `containers/`: [Stateful][dumb-comp] react components. These are usually top-level route handlers.
 - `components/`: [Stateless/Dumb][dumb-comp] react components. These guys determininstically render based on props.
 
+
+### Configuration Values
+
+Configuration values are stored in the `config/` directory. You gain access to the configuration values by importing the config object from `config/index.js`. The configuration values are loaded as such:
+
+1. The values in `config/default.js`
+2. The values in `config/{{APP_ENV}}.js` where `APP_ENV` is an environment variable.
+3. If `APP_ENV` is not set, `config/development.js` will be loaded by default.
+
+[the logic for loading the config is here](https://github.com/UTD-CRSS/app.exploreapollo.org/blob/master/config/index.js).
+
+#### Example
+
+We use the configuration file to determine the API entry point in our actions.
+
+1. [import the configuration object](https://github.com/UTD-CRSS/app.exploreapollo.org/blob/a999375d881ed763fd18b852b402582b5a6e647b/src/actions/index.js#L5)
+2. [use the configuration object](https://github.com/UTD-CRSS/app.exploreapollo.org/blob/a999375d881ed763fd18b852b402582b5a6e647b/src/actions/index.js#L31)
+3. If the `APP_ENV` is set to `staging`, [this](https://github.com/UTD-CRSS/app.exploreapollo.org/blob/a999375d881ed763fd18b852b402582b5a6e647b/config/staging.js#L3) will be the value for `config.apiEntry`.
+
+Consider the following example commands:
+
+```bash
+npm run dev                     # runs dev server with development API
+APP_ENV=staging npm run dev     # runs dev server with staging API
+APP_ENV=production npm run dev  # runs dev server with production API
+```
+
 [nvm]: https://github.com/creationix/nvm
 [jest]: https://facebook.github.io/jest/
 [dumb-comp]: https://github.com/uberVU/react-guide/blob/master/props-vs-state.md#component-types