Create DOAP(Description of a Project) file

Apache REEF is registered in Apache Project Directory.

https://projects.apache.org/projects.html?number
(Projects by number of committers)

However, when we search by Category, REEF is missing like the following.

https://projects.apache.org/projects.html?category

Apache REEF project should exist under 'big-data' category.

To provide that information, we need to make DOAP(Description of a Project, https://en.wikipedia.org/wiki/DOAP) file. Since DOAP files are RDF files, they are usually parts of Web page.

http://hadoop.apache.org/doap.rdf
http://spark.apache.org/doap.rdf

This issue addresses creating DOAP file.