Clarify the README that this is an IDL project and no binaries are distributed

[codefromthecrypt]

In the last release vote, it seemed evident the README could be clarified in a couple ways.

• beyond examples, further clarify that proto and swagger in this project are inputs to other tools
  • ex. point to the main docs of the IDL compiler
  • note that most users will not use these directly as tracing libraries will
• address a frequently asked question which is "why is there a maven build?"
  • this is for java projects convenience to depend on the protos.
  • this is not a binary
  • this is an alternative to the source distribution, that only includes the proto file