[Android] Place README.md in empty src of xwalk_core_library

Otherwise, the archiving will ignore the empty src folder.
The build.xml in android sdk using ant to create apk requires
a src folder even it's empty.

BUG=XWALK-1650, XWALK-1654
Merge pull request #1930 from wang16/add_readme_in_core_library

[Android] Place README.md in empty src of xwalk_core_library