[libgpiod][RFC/RFT 17/18] README: document the DBus API

[Bartosz Golaszewski <brgl@xxxxxxxx>]

From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>

Add information on the DBus API as well as gpio-manager and gpiocli to
the README file.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
---
 README | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/README b/README
index 8e1e1c7..54f2df5 100644
--- a/README
+++ b/README
@@ -224,6 +224,70 @@ C library using make, they will be automatically configured to build against the
 build results of the C library. Please refer to bindings/rust/libgpiod/README.md
 for more information.
 
+DBUS
+----
+
+A commonly requested feature for the GPIO character device was state persistence
+after releasing the lines (as a kernel feature) or providing a central authority
+(in user-space) that would be in charge of keeping the lines requested and in a
+certain state (similarily to how the sysfs ABI works). DBus API has been
+provided to address this requirement. We define an interface covering the
+majority of the GPIO chardev's functionality and implement it from both the
+server and client sides in the form of the gpio-manager daemon and the gpiocli
+command-line utility for talking to the manager.
+
+DBus support can be built by passing --enable-dbus to configure. The daemon
+is bundled with a systemd unit file and an example configuration file for the
+io.gpiod1 interface that allows all users to access basic information about the
+GPIOs in the system but only root to request lines or change their values.
+
+With the manager running the user can run gpiocli to control GPIOs by asking
+gpio-manager to act on their behalf:
+
+    # Detect chips in the system.
+    $ gpiocli detect
+    gpiochip0 [INT34C6:00] (463 lines)
+
+    # Request a set of lines. Note that gpiocli exits immediately but the
+    # state of the lines is retained because it's the gpio-manager that
+    # requested them.
+    $ gpiocli request --output foo=active
+    request0
+
+    # Previous invocation printed out the name of the request by which the
+    # caller can refer to it later. All active requests can also be inspected
+    # at any time.
+    $ gpiocli requests
+    request0 (gpiochip1) Offsets: [5]
+
+    # We can print the information about the requested line using the
+    # information above.
+    $ gpiocli info --chip=gpiochip1 5
+    gpiochip1   5:	"foo"		[used,consumer="gpiocli request",managed="request0",output,push-pull]
+
+    # We can now change the value of the line.
+    $ gpiocli set foo=inactive
+
+    # And read it.
+    $ gpiocli get foo
+    "foo"=inactive
+
+    # We can even reconfigure it to input and enable edge-detection.
+    $ gpiocli reconfigure --input --both-edges request0
+
+    # And wait for edge events.
+    $ gpiocli monitor cos
+    21763952894920 rising  "foo"
+
+    # And finally release the request.
+    $ gpiocli release request0
+
+For more information please refer to the output of gpiocli --help as well as
+gpiocli <command> --help which prints detailed info on every available command.
+
+Of course - this being DBus - users can talk to gpio-manager using any DBus
+library available and are not limited to the provided client.
+
 TESTING
 -------
 
-- 
2.40.1