Clean up Communication Section of the Readme

The [communication header of the readme](https://github.com/beckn/protocol-specifications#communication) could be cleaned up a little. I'm new to the protocol, so I was reading through to get a better idea of it.

At this point in the readme, there's a list of "types of communication" between a "BAP, BG, BPP" etc. These aren't familiar to me, and a quick search only brings me back here. Rather than using acronyms, these should be described, or maybe left to more specific documentation and linked to here.

Also, the last paragraph is essentially a repeat of the first. Both say the protocol is asynchronous, so this should be consolidated.

Overall very interesting project, I'm intrigued! I hope this feedback is well received.