LRDOCS-1392 Build .METADATA file for the ZIPs

On running 'ant dist', the Ant buildfile constructs a base URL from your GitHub settings (see build.properties) and writes the base URL property to a file named .METADATA in the resulting dist/*.zip file. The KB portlet uses the base source URL to construct the source URL for each of the KB articles that are imported from the .zip file.

https://issues.liferay.com/browse/LRDOCS-1392