[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

(cherry-pick from commit e5d674d79)
Merge pull request #1932 from wang16/crosswalk-6

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