gnu: ddcutil: Install udev rules at their expected location.

* gnu/packages/hardware.scm (ddcutil) [arguments]: New field.
[description]: Document how the included udev rules can be used.

gnu/packages/hardware.scm

@@ -491,6 +491,22 @@ (define-public ddcutil
        (sha256
         (base32 "0hm0cm4m4hk1jjy7kddg613mynvwlii3kp8al0j9v3c6mcx3p4mx"))))
     (build-system gnu-build-system)
+    (arguments
+     (list
+      #:phases
+      #~(modify-phases %standard-phases
+          (add-after 'install 'install-udev-rules
+            (lambda* (#:key outputs #:allow-other-keys)
+              ;; Move the udev rules to their expected location in Guix
+              ;; System, so they can be more easily used.
+              (let ((rules.d (string-append #$output "/lib/udev/rules.d")))
+                (mkdir-p (dirname rules.d))
+                (rename-file (string-append #$output "/share/ddcutil/data")
+                             rules.d)
+                ;; Patch a reference to the ddcutil command.
+                (substitute* (string-append rules.d "/45-ddcutil-usb.rules")
+                  (("/usr/bin/ddcutil")
+                   (search-input-file outputs "bin/ddcutil")))))))))
     (native-inputs
      (list pkg-config))
     (inputs
@@ -498,9 +514,9 @@ (define-public ddcutil
            glib
            kmod
            i2c-tools
-           libdrm ; enhanced diagnostics
-           libusb ; support USB monitors
-           libx11 ; enhanced diagnostics
+           libdrm                       ;enhanced diagnostics
+           libusb                       ;support USB monitors
+           libx11                       ;enhanced diagnostics
            libxrandr
            zlib))
     (home-page "https://www.ddcutil.com/")
@@ -519,7 +535,11 @@ (define-public ddcutil
 One particular use case is in colour profile management.  Monitor calibration
 is relative to the monitor colour settings currently in effect, e.g. red gain.
 ddcutil allows colour-related settings to be saved at the time a monitor is
-calibrated, and restored when the calibration is applied.")
+calibrated, and restored when the calibration is applied.
+
+This package includes udev rules that can be used by adding this package to
+the @code{rules} field of the @code{udev-configuration} record.  It gives
+read/write access to i2c devices to users in the @samp{i2c} group.")
     (license (list license:bsd-3        ; FindDDCUtil.cmake
                    license:gpl2+))))    ; everything else