[#750] Add sonatype snapshot repository to readme information
[#750] Add sonatype snapshot repository to readme information

* fixes #750 
dbs-leipzig/gradoop,rostam/gradoop,smee/gradoop,niklasteichmann/gradoop,niklasteichmann/gradoop,dbs-leipzig/gradoop,galpha/gradoop,galpha/gradoop,rostam/gradoop,smee/gradoop