Add schemas for SIR, SOR  and CSW-ebRIM; add readme

This pull request adds discussion paper schemas for SIR and SOR and a module for CSW-ebRIM based on an existing schema file.

Also, basic documentation is added in Readme.md.

`mvn install` works both locally and on Travis CI (https://travis-ci.org/nuest/common-xml), but `mvn test` gives an error - so feedback is welcome.