Document how to setup development environment (for contributors)

[lll000111]

Whatever I do I can't get the IDE (Android Studio,latest version) to recognize all packages used by the Android code of this package.

I managed to get rid of errors for "okhttp3" related imports by adding compile 'com.squareup.okhttp3:okhttp:+' to build.gradle's "dependencies" section, but I still get "Cannot resolve symbol" for "com.facebook.react.modules.network.CookieJarContainer", "com.facebook.react.modules.network.TLSSocketFactory", "okhttp3.JavaNetCookieJar".

By the way, line https://github.com/wkh237/react-native-fetch-blob/blob/master/android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java#L698, the IDE says "sslSocketFactory is deprecated" -- also see https://stackoverflow.com/questions/31002159/now-that-sslsocketfactory-is-deprecated-on-android-what-would-be-the-best-way-t

I added the code shown there to the RNFetchBlobReq.java function and while I don't know if it works at least I see no complains, I'll submit it in an upcoming PR but it needs to be tested.

[lll000111]

Let's have this issue on RNFB-dev instead.