README.org: Don't recommend load-broadcom-sta-service.

Closes #26.

* README.org: Replace `load-broadcom-sta-service` example with new
`kernel-loadable-modules` field.
* nongnu/services/kernel-modules.scm (load-broadcom-sta-service): Update
docstring to recommend switching to `kernel-loadable-modules`.

README.org

@@ -133,20 +133,16 @@ addition to firmware. To use such hardware you will also need to add a service
 to load that module on boot, and blacklist conflicting kernel modules:
 
 #+BEGIN_SRC scheme
-  (use-modules (nongnu packages linux)
-               (nongnu services kernel-modules))
+  (use-modules (nongnu packages linux))
 
   (operating-system
     (kernel linux)
     ;; Blacklist conflicting kernel modules.
     (kernel-arguments '("modprobe.blacklist=b43,b43legacy,ssb,bcm43xx,brcm80211,brcmfmac,brcmsmac,bcma"))
+    (kernel-loadable-modules (list broadcom-sta))
     (firmware (cons* broadcom-bt-firmware
                      %base-firmware))
-    ...
-    (services
-     (cons* (load-broadcom-sta-service)
-            ...
-            %desktop-services)))
+    ...)
 #+END_SRC
 
 * Contributing

nongnu/services/kernel-modules.scm

@@ -1,4 +1,4 @@
-;;; Copyright © 2019 Alex Griffin <a@ajgrf.com>
+;;; Copyright © 2019, 2020 Alex Griffin <a@ajgrf.com>
 ;;;
 ;;; This program is free software: you can redistribute it and/or modify
 ;;; it under the terms of the GNU General Public License as published by
@@ -66,6 +66,8 @@
 
 (define* (load-broadcom-sta-service #:key (broadcom-sta broadcom-sta))
   "Return a service that loads the nonfree Broadcom wireless driver.
+This function is here only for backwards compatibility.  You should prefer
+the `kernel-loadable-modules' mechanism to this now.
 
 Users should also blacklist conflicting modules by adding the following
 to kernel-arguments: