Update README for NIO based implementation (#483)
Update README for NIO based implementation (#483)

* Update README for NIO based implementation

Motivation:

The README was focused mostly on the libgrpc implementation.

Modifications:

Rewrote large chunks of the README, including:
- How to integrate GRPC with an existing/new project
- Details on how to use GRPC with an example
- Differences between the call types
- How to make calls without a generated client

Result:

More up-to-date README.

* Fix typos.

* Add TLS section.

grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift