Create new top level README

[banool]

The README should:

• Tell users to look at each subdirectory for development instructions.
• Describe briefly what the project is, how it came to be, etc.
• Explain the ownership and contribution model (e.g. Aptos Labs is giving this to the community).
• More general vision stuff.

[banool]

The README should also discuss high level direction not captured in the GH issues. For example, the general question of whether we really want to create our own chat server or if it'd be better to use some paid third party solution. The answers to those questions depend on which parts of the community are interested, how legitimate we want to make this, etc.

It'd be worth mentioning that really this is two separate projects, a chat server with Aptos auth (and potentially Aptos account based message encryption) and then chat "plugins" such as hong bao.