Update readme with developer instructions

Using [this old readme](https://github.com/adamantivm/rosjava_build_tools/blob/f5366da3a62963ad16e44cf0d13e303e4d80344c/README.md) as a starting point, add developer instructions in the repository.

Wise suggestions by @stonier [here](https://github.com/rosjava/rosjava_build_tools/pull/26#issuecomment-267256557)