docs: point to facebook/flow in readme

[FireyFly]

I'm not entirely sure why the base flow repository is under the facebook organisation rather than the flowtype one, but it would be great if the Readme at least points to the repo containing the source code that this is a binary-only release of.

[urmil22]

@FireyFly @mroch Could you please explain this issue clearly? I’d like to work on that.

[FireyFly]

Flow lives at https://github.com/facebook/flow/ rather than (as one might naively assume) under the https://github.com/flow organization. It appears that six years later there's still no link to the flow repo in the readme, it just points to the flow.org website. This makes it unnecessarily hard to find the source code from this binary build repo.

[urmil22]

@mroch @FireyFly Can you assign this to me?

[FireyFly]

I don't think I have the necessary permissions to (but looks like you already opened a PR that'd address it)