Update installation instructions for NixOS (abraunegg#981)

* Update installation instructions for NixOS

docs/INSTALL.md

@@ -6,7 +6,7 @@ This project has been packaged for the following Linux distributions:
 *   Arch Linux, available from AUR as [onedrive-abraunegg](https://aur.archlinux.org/packages/onedrive-abraunegg/)
 *   Debian, available from the package repository as [onedrive](https://packages.debian.org/sid/net/onedrive)
 *   Fedora, available via package repositories as [onedrive](https://koji.fedoraproject.org/koji/packageinfo?packageID=26044)
-*   NixOS, available on unstable channel (and stable since 20.03). Use package `onedrive` either by adding it to `configuration.nix` or by using the command `nix-env -iA <channel name>.onedrive`. This does not install a service. There is a [PR](https://github.com/NixOS/nixpkgs/pull/77734) (which needs work) which has code which can be used to install a service. See documentation in the same PR.
+*   NixOS, use package `onedrive` either by adding it to `configuration.nix` or by using the command `nix-env -iA <channel name>.onedrive`. This does not install a service. To install a service, use unstable channel (will stabilize in 20.09) and add `services.onedrive.enable=true` in `configuration.nix`. You can also add a custom package using the `services.onedrive.package` option (recommended since package lags upstream). Enabling the service installs a default package too (based on the channel). You can also add multiple onedrive accounts trivially, see [documentation](https://github.com/NixOS/nixpkgs/pull/77734#issuecomment-575874225)`.
 *   openSUSE, available for Tumbleweed as [onedrive](https://software.opensuse.org/package/onedrive) - just install using: `zypper in onedrive`
 *   Slackware, available from the slackbuilds.org repository as [onedrive](https://slackbuilds.org/repository/14.2/network/onedrive/)
 *   Solus, available from the package repository as [onedrive](https://dev.getsol.us/search/query/FB7PIf1jG9Z9/#R)