Move docs to docsify

docs/usage.md

@@ -0,0 +1,36 @@
+# Usage
+
+After creating a Firebase project and installing the library, we can use it in our project by importing the library in our JavaScript:
+
+```javascript
+import RNFirebase from 'react-native-firebase'
+```
+
+We need to tell the Firebase library we want to _configure_ the project. RNFirebase provides a way to configure both the native and the JavaScript side of the project at the same time with a single command:
+
+```javascript
+const firebase = RNFirebase.initializeApp({
+  // config options
+});
+```
+
+## Configuration Options
+
+| option           | type | Default Value           | Description                                                                                                                                                                                                                                                                                                                                                      |
+|----------------|----------|-------------------------|----------------------------------------|
+| debug | bool | false | When set to true, RNFirebase will log messages to the console and fire `debug` events we can listen to in `js` |
+| persistence | bool | false | When set to true, database persistence will be enabled. |
+
+For instance:
+
+```javascript
+import RNFirebase from 'react-native-firebase';
+
+const configurationOptions = {
+  debug: true
+};
+
+const firebase = RNFirebase.initializeApp(configurationOptions);
+
+export default firebase;
+```