add missing write-text.nix file

modules/lib/write-text.nix

@@ -0,0 +1,55 @@
+{ lib, mkTextDerivation }:
+
+{ config, name, ... }:
+
+with lib;
+
+let
+
+  drv = mkTextDerivation name config.text;
+
+in
+
+{
+  options = {
+
+    enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether this file should be generated.
+        This option allows specific files to be disabled.
+      '';
+    };
+
+    text = mkOption {
+      type = types.lines;
+      default = "";
+      description = ''
+        Text of the file.
+      '';
+    };
+
+    target = mkOption {
+      type = types.str;
+      default = name;
+      description = ''
+        Name of symlink.  Defaults to the attribute name.
+      '';
+    };
+
+    source = mkOption {
+      type = types.path;
+      description = ''
+        Path of the source file.
+      '';
+    };
+
+  };
+
+  config = {
+
+    source = mkIf (config.text != "") (mkDefault drv);
+
+  };
+}