Cleanup of the README

[DImuthuUpe]

The current README is not helpful for external users who come to understand how the project works. We need to make is simple and provide general information on the project.

[DImuthuUpe]

Initial draft is available at https://github.com/apache/airavata-mft/commit/50c7f27f4c61ad17f621317eb86dc0410425950e