Transfer to NixOS GitHub organisation

[infinisil]

As outlined in #153, this repository needs to be transferred to the official NixOS organisation.

Action items:

• @Sereja313:
  • Please give repo admin access to @zimbatm, a NixOS org owner who can do the transfer
• @zimbatm:
  • Please add @Sereja313 to the NixOS organisation and add them to the formatting team
  • Transfer this repository to https://github.com/NixOS/nixfmt
  • Give @infinisil and @piegamesde repository admin access who can then make further permission changes as necessary, including giving the formatter team write access

[Sereja313]

i sent an invitation to @zimbatm

[zimbatm]

done!