Remove / tone down the scary 'NOT MAINTAINED' header

[ArturGajowy]

The README says:

Note: Nailgun is based on original code developed by Marty Lamb. In October, 2017, Marty transferred the repository to Facebook, where it is currently maintained by Buck team. Nailgun will remain available under the Apache license, version 2.0.

I don't know why, but I always assumed this is not the repo on which nailgun is currently developed. I know the text doesn't say that, but the header-size of that excerpt screams - at least to me - louder than the actual text.

Wouldn't a Origins/Credits section at the bottom of the README suffice? It'd help the likes of me who can't read recognize that this is nailgun's home, and that it is indeed maintained :)