Add remote config documentation

docs/modules/config.md

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+# Remote Config
+
+?> Remote Config is not available on the web SDK. Please read the [documentation](https://firebase.google.com/docs/remote-config) on when & why Remote Config benefits your app.
+
+## API
+
+### enableDeveloperMode()
+
+Enable Remote Config developer mode to allow for frequent refreshes of the cache.
+
+```js
+firebase.config().enableDeveloperMode();
+```
+
+### setDefaults(defaults: `Object`)
+
+Sets default values for the app to use when accessing values. Any data fetched and activated will override any default values.
+Any values in the defaults but not on Firebase will be untouched.
+
+```js
+ firebase.config().setDefaults({
+   hasExperimentalFeature: false,
+ });
+```
+
+### fetch(): `Promise<String>`
+
+Fetches the remote config data from Firebase, defined in the dashboard. To have fetched data available in the active config, use
+`activateFetched`.
+
+// TODO Errors
+
+```js
+ firebase.config()
+   .fetch()
+   .then(() => {
+     console.log('Successfully fetched data');
+   })
+   .catch((error) => {
+     console.error('Unable to fetch remote config data: ', error);
+   });
+```
+
+### fetchWithExpirationDuration(duration: `int`): `Promise<String>`
+
+Same as `fetch`, however defines (in seconds) how long fetched data is available in the active config.
+
+Default duration is 12 hours (43200 seconds)*[]:
+
+
+```js
+ firebase.config()
+   .fetchWithExpirationDuration(1337)
+   .then(() => {
+     console.log('Successfully fetched data for 1337 seconds');
+   })
+   .catch((error) => {
+     console.error('Unable to fetch remote config data: ', error);
+   });
+```
+
+### activateFetched(): `Promise<Boolean>`
+
+Moves fetched data in the apps active config. Always successfully resolves with a boolean value.
+
+```js
+ firebase.config()
+   .activateFetched()
+   .then((success) => {
+     if (success) {
+       console.log('Successfully activated fetched config into active config');
+     } else {
+       console.log('No fetched config was found or fetched config is already active');
+     }
+   });
+```
+
+### getValue(key: `String`): `Promise<Object>`
+
+Gets a config item by key. Returns an object containing source (default, remote or static) and val function.
+
+```js
+ firebase.config()
+   .getValue('foobar')
+   .then((snapshot) => {
+     console.log('Got value from source: ', snapshot.source);
+     console.log('Value of foobar: ', snapshot.val());
+   })
+   .catch(console.error);
+```
+
+### getValues(keys: `Array<String>`): `Promise<Object>`
+
+Gets multiple values by key. Returns an object of keys with the same object returned from `getValue`.
+
+```js
+ firebase.config()
+   .getValues(['foobar', 'barbaz'])
+   .then((snapshot) => {
+     console.log('Value of foobar: ', snapshot.foobar.val());
+     console.log('Value of barbaz: ', snapshot.barbaz.val());
+   })
+   .catch(console.error);
+```
+
+## Usage
+
+```js
+if (__DEV__) {
+  firebase.config().enableDeveloperMode();
+}
+
+// Set default values
+firebase.config().setDefaults({
+  hasExperimentalFeature: false,
+});
+
+firebase.remoteConfig.fetch()
+  .then(() => {
+    return firebase.remoteConfig().activateFetched();
+  })
+  .then((activated) => {
+    if (!activated) console.log('Fetched data not activated');
+    return firebase.remoteConfig().getValue('hasExperimentalFeature');
+  })
+  .then((snapshot) => {
+    const hasExperimentalFeature = snapshot.val();
+
+    if(hasExperimentalFeature) {
+      // enableSuperCoolFeature();
+    }
+
+    // continue booting app
+  })
+  .catch(console.error);
+```