Add notice regarding use case of remote builder (Mic92#34)

* Add notice regarding use case of remote builder

* README.md: improve remote builder documentation

Co-authored-by: Jörg Thalheim <joerg@thalheim.io>

README.md

@@ -108,12 +108,14 @@ To increase the size of the virtual hard drive, i. e. times 20 (see [virtualisat
 Notice that for this option to become effective you may also need to delete previous block device files created by qemu (`nixos.qcow2`).
 
 Notice that changes in the nix store are written to an overlayfs backed by tmpfs rather than the block device
-that is configured by `virtualisation.diskSize`. This tmpfs can be changed however by using:
+that is configured by `virtualisation.diskSize`. This tmpfs can be disabled however by using:
 
 ```nix
 { virtualisation.writableStoreUseTmpfs = false; }
 ```
 
+This option is recommend if you plan to use nixos-shell as a remote builder.
+
 ## Graphics/Xserver
 
 To use graphical applications, add the `virtualisation.graphics` NixOS option (see `examples/vm-graphics.nix`).