Create a `specification/` for `.md` documents with what we agreed upon

[jorgecarleitao]

I would like to propose that we create a directory specification/ where we drop .md files that result from the discussions we have over google docs, so that they are given the proper visibility in this repository.

We would continue to use google docs to iterate on them. Once ready, I propose to PR them to this repo.

Reasons:

1. Licensing: the work on them imo should remain ASF-licensed
2. visibility: we have a simple place where we can refer to
3. easier to contribute: it is easy propose changes to those documents
4. audit: changes to those documents are properly versioned, specially after we "agreed" on them

[jorgecarleitao]

Any thoughts @alamb @andygrove @houqp @Dandandan @nevi-me ?

[houqp]

+1000, could we also serve them as part of datafusion's user/developer documentation?

[Dandandan]

I think it is a good idea. What about using the term RFC?

[alamb]

I think it is a great idea. I vaguely remember that Fernando suggested a more full featured version of a process to create the documents as well, but perhaps starting with recording the docs we do make is a good step towards formalizing the process more

In other words, 👍