Add local podspec
Add local podspec

Allow installing native part of the library in a way similar to
`react-native` by adding a line to a `Podfile` (assume you have your
`node_modules` in the same directory):

```
pod 'RNSound', :path => './node_modules/react-native-sound'
```

hayanmind/react-native-sound,zmxv/react-native-sound,hayanmind/react-native-sound,zmxv/react-native-sound,zmxv/react-native-sound,calvium/react-native-sound-exoplayer,gvenk/react-native-sound,calvium/react-native-sound-exoplayer,hayanmind/react-native-sound,gvenk/react-native-sound,gvenk/react-native-sound,zmxv/react-native-sound,calvium/react-native-sound-exoplayer,gvenk/react-native-sound,hayanmind/react-native-sound