added overview to README

ur_rtde_driver/README.md

@@ -1,4 +1,47 @@
 # ur_rtde_driver 
 
-The actual driver package. For more information, please see the README in the top-level folder of
+This package contains the actual driver for UR robots. It is part of the *universal_robots_driver*
-this repository.
+repository and requires other packages from that repository. Also, see the [main repository's
+README](../README.md) for information on how to install and startup this driver.
+
+## Technical details
+The following image shows a very coarse overview of the driver's architecture.
+
+![Architecture overview](doc/architecture_coarse.svg "Architecture overview")
+
+Upon connection to the primary interface the robot sends version and calibration information which
+is consumed by the *calibration_check*. If the calibration reported by the robot doesn't match the
+one configured (See TODO: Insert Link) an error will be printed to Roslog.
+
+Real-time data from the robot is read through the RTDE interface. This is done automatically as soon
+as a connection to the robot could be established. Thus joint states and IO data will be immediately
+available.
+
+To actually control the robot, a program node from the **External Control** URCap must be running on
+the robot interpreting commands sent from an external source. When this program is not running, no
+controllers moving the robot around will be available. Please see the [initial setup
+guide](../README.md) on how to install and start this on the robot.
+
+The URScript that will be running on the robot is requested by the **External Control** program node
+from the remote ROS PC. The robot *ur_control.launch* file has a parameter called `urscript_file` to
+select a different program than the default one that will be sent as a response to a program
+request.
+
+**Custom script snippets** can be sent to the robot on a topic basis. By default, they will
+interrupt other programs (such as the one controlling the robot). For a certain subset of functions,
+it is however possible to send them as secondary programs. See [UR
+documentation](https://www.universal-robots.com/how-tos-and-faqs/how-to/ur-how-tos/secondary-program-17257/)
+on details.
+<br/>
+**Note to e-Series users:**
+The robot won't accept script code from a remote source unless the robot is put into
+*remote_control-mode*. However, if put into *remote_control-mode*, the program containing the
+**External Control** program node can't be started from the panel. In the future, this will be
+supported in the *headless* mode which doesn't require a panel being connected. Until that, a
+possible workaround is to use a third party dashboard client (e.g. [ur_dash](https://github.com/gocarter/ur_dash))
+to start the program through the dashboard server.
+
+For using the **tool communication interface** on e-Series robots, a `socat` script is prepared to
+forward the robot's tool communication interface to a local device on the ROS PC. See [the tool
+communication setup guide](doc/setup_tool_communication.md) for details.
+