README.md: Reorganize and trim
README.md: Reorganize and trim

Reorganize the README to flow better for new users, pointing them in
good directions. Also try to reduce the size so they actually read the
thing. It's still to long, but is a step in the right direction.

Fixes #4615
zhangkun83/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,zhangkun83/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,zhangkun83/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java,zhangkun83/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,carl-mastrangelo/grpc-java,ejona86/grpc-java