## What is the purpose of the change

Specification for the new `x/streamswap` module.

## Brief Changelog

* spec doc as README.md file
* updated comment in streamswap proto file.

## Documentation and Release Note

  - Does this pull request introduce a new feature or user-facing behavior changes? 
      yes - new x/streamswap API. However it's not wired in this PR so I suggest to put it in a changelog once thing will be wired up.