Added instructions for compressing single file

I think a lot of people have had problem with not being able to compress a single file and then looking at the issue 240(#240 (comment)) to find a solution, and since the problem has been around for a long time, I think it would be nice to add a note to the readme to be friendly to new users ;)

Author: YaoHuiJi

README.md

@@ -53,6 +53,8 @@ import { MainBundlePath, DocumentDirectoryPath } from 'react-native-fs'
 
 ***NOTE: only support zip folder, not file entries***
 
+***NOTE: the string version of source is for folder, the string[] version is for file, so you want to zip a single file, use zip([file]) instead of zip(file)***
+
 Example
 
 ```js
@@ -74,6 +76,8 @@ zip(sourcePath, targetPath)
 
 ***NOTE: only support zip folder, not file entries***
 
+***NOTE: the string version of source is for folder, the string[] version is for file, so you want to zip a single file, use zip([file]) instead of zip(file)***
+
 ***NOTE: encryptionType is not supported on iOS yet, so it would be igonred on that platform.***
 
 Example