No podspec found for `RNFS` in `./node_modules/react-native-fs`

I run "npm install react-native-fs --save" first, then
I follow the instrction and create a Podfile:

```
platform :ios, '8.0'
use_frameworks!

target 'testFile' do
pod 'RNFS', :path => './node_modules/react-native-fs'
end
```

then:
pod install

the output is:

```
Analyzing dependencies
Fetching podspec for `RNFS` from `./node_modules/react-native-fs`
[!] No podspec found for `RNFS` in `./node_modules/react-native-fs`
```

I delete "use_frameworks!" in Podfile, it still doesn't work.