doc: add dialup-overview.txt

diff --git a/doc/dialup-overview.txt b/doc/dialup-overview.txt
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+DialUp Networking diagram
+=========================
+
+This diagram explains how oFono and ConnMan are interacting to
+handle a data call from a DUN client.
+1) GAtServer receive ATD*99#.
+2) The GAtPPP server is setup.
+3) oFono through GAtPPP notify ConnMan that he needs a TUN/TAP interface.
+4) oFono is notified that TUN/TAP interface is created and can start exchanging
+PPP packets with DUN client. Those PPP packets are converted into IP stream
+and transmitted to/received from TUN/TAP interface.
+5) ConnMan is selecting which interface to send IP packets (WiFi, 3G, ethX, ...).
+
+*----------------*            *----------------*
+|                |  ATD*99#   |                |
+|   DUN Client   |----------->|    GAtServer   |
+|                |<---*       |                |
+*----------------*    |       *----------------*
+                      |             |
+              PPP     |             | setup_ppp()
+              Packets |             |
+                      |             |
+                      |             V           Need
+                      |    *----------------*   /dev/net/tun *----------------*
+                      *--->|                |   interface    |                |
+                           |     GAtPPP     |<-------------->|     ConnMan    |
+                      *--->|                |   interface    |                |
+                      |    *----------------*   created      *----------------*
+              IP      |                                          |
+              Stream  |                                          |Create
+                      |    *------------------------*            |/dev/net/tun
+                      |    |                        |            |interface
+                      *--->|    TUN/TAP interface   |<-----------*
+                           |                        |
+                           *------------------------*
+                                      IP | Stream
+                             routed with | ConnMan rules
+                   *---------------------*--------------------*
+                   |                     |                    |
+                   |                     |                    |
+                   V                     V                    V
+            *-------------*       *-------------*      *-------------*
+            |             |       |             |      |             |
+            |  WiFi       |       |   Modem     |      |    EthX     |
+            |             |       |             |      |             |
+            *-------------*       *-------------*      *-------------*
+