add pod install to readme (#306)

* Update README.md

* Update README.md

* Update README.md

Author: rjruizes

README.md

@@ -34,7 +34,14 @@ npm install --save @react-native-community/blur
 react-native link @react-native-community/blur
 ```
 
-3. (Android only) Add the following to `android/app/build.gradle`
+3. (iOS only) Install to Xcode:
+
+```
+cd ios
+pod install
+```
+
+4. (Android only) Add the following to `android/app/build.gradle`
 
 ```
 android {
@@ -50,7 +57,7 @@ android {
 }
 ```
 
-4. (Android only, optional) 
+5. (Android only, optional) 
 If you've defined _[project-wide properties](https://developer.android.com/studio/build/gradle-tips.html)_ (**recommended**) in your root `build.gradle`, this library will detect the presence of the following properties:
 
 ```groovy
@@ -67,13 +74,13 @@ ext {
 }
 ```
 
-5. Include the library in your code:
+6. Include the library in your code:
 
 ```javascript
 import { BlurView, VibrancyView } from "@react-native-community/blur";
 ```
 
-6. Compile and have fun!
+7. Compile and have fun!
 
 ### BlurView