Add manual linking instructions

There are a million issues around people having issues with automatically linking and having a link to manual instructions would address a lot of these up front. This also follows the instructions pattern of most react native modules.

Bug Fix: getExternalFilesDir null check

Update README with proper status code handling

Fix README.md

Fix README

Fix example links for Download and Multipart/form-data

remove extra space

createJSModules was removedin React Native 0.47.0 and results in the attached Build Error.
removing @Override fixes build (didn't remove function because I don't know if it should be kept for downard compatability?)

```
node_modules/react-native-fetch-blob/android/src/main/java/com/RNFetchBlob/RNFetchBlobPackage.java:23: error: method does not override or implement a method from a supertype
    @Override
    ^
1 error

Incremental compilation of 1 classes completed in 0.219 secs.
:react-native-fetch-blob:compileReleaseJavaWithJavac FAILED

FAILURE: Build failed with an exception.
```

To support the default react-native gradle version
https://github.com/wkh237/react-native-fetch-blob/pull/396

Change to jCenter because mavenCentral does not have the latest version of the package