Document how to build JDBC driver, Java components outside of CMake

[andrewseidl]

We currently rely on CMake to generate the Thrift bindings and to set a few variables required to build the Java components (Thrift version, now the package version).

There have been a few requests (one at heavyai/heavydb#274 (comment)) asking how to build these components separately. It's possible, just need to update the README with how to do so.

Relevant sections of CMake:

https://github.com/omnisci/mapd-core/blob/75853e87efd32e26ad1270a59745a2b33269bfde/CMakeLists.txt#L649

https://github.com/omnisci/mapd-core/blob/75853e87efd32e26ad1270a59745a2b33269bfde/CMakeLists.txt#L439-L455 (ideally we'd move the thrift -gen java to something managed by maven itself)

[randyzwitch]

Closing this, since the repos have now been split