Make clear sub is an extension to pub

[fluffy]

No description provided.

[vasilvv]

I suggest roughly the following text:

"The working group will focus on defining two protocols:

• A protocol for point-to-point publication of media
• A protocol, or extensions to the protocol above, that allows the receiving party to control what media is being published"

I actually think this is roughly equivalent to the problem we're solving in #31 , at least it definitely overlaps.

[suhasHere]

@vasilvv does the latest master text looks fine or do you think we should be explicit on the distinction ?