Create README.md from Wiki content (#342)

Co-authored-by: Alejandro Hernández Cordero <[email protected]>

polled_camera/README.md

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+## `polled_camera` Overview
+
+`polled_camera` defines the ROS interface that client nodes use to request images from a polling camera driver node (e.g. [prosilica_camera](https://index.ros.org/p/prosilica_camera/)). The protocol is:
+
+* The camera driver advertises a service call `<camera>/request_image` of type [polled_camera/GetPolledImage](https://github.com/ros-perception/image_common/blob/rolling/polled_camera/srv/GetPolledImage.srv).
+* The client calls the service, specifying an output namespace.
+* On receiving a request, the driver captures an image and returns its time stamp in the service response.
+* The driver publishes (latching) the [sensor_msgs/Image](http://www.ros.org/doc/api/sensor_msgs/html/msg/Image.html) and [sensor_msgs/CameraInfo](http://www.ros.org/doc/api/sensor_msgs/html/msg/CameraInfo.html) to `<response_namespace>/image_raw` and `<response_namespace>/camera_info`.
+* Clients subscribe to the response topics just like any other camera image stream.
+* When a disconnection causes the number of subscribers to drop to zero, the publisher shuts down.
+
+See the [code API](http://www.ros.org/doc/api/polled_camera/html/) (unstable) for more information on writing polled camera drivers or clients.
+
+There are several reasons for publishing images to topics instead of returning them from the service call:
+
+* **Consistency:** Client-side processing is always performed in an image callback, regardless of what type of camera is used.
+* **Visibility:** Topics can be monitored in [image_view](https://index.ros.org/p/image_view/) or [rviz](https://index.ros.org/p/rviz/).
+* **Reproducibility:** Topics can be [bagged](https://index.ros.org/p/rosbag2/) and played back later.
+* **Compression:** Can take advantage of [image_transport](https://index.ros.org/p/image_transport/) to compress images, if desired.
+
+## Nodes
+
+### poller
+
+Continually requests images from a polled camera at a specified rate in Hz, making the camera look like it is capturing continuously.
+
+Usage:
+
+```bash
+# Poll "my_camera" at 5 Hz, publishing in namespace my_polled_output/.
+$ poller 5 camera:=my_camera output:=my_polled_output
+```
+
+Published Topics
+* `<output>/image_raw` ([sensor_msgs/Image](https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/Image.msg))
+  Image topic, actually published by the camera driver. output should be remapped by the user.
+* `<output>/camera_info` ([sensor_msgs/CameraInfo](https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/CameraInfo.msg))
+  Info topic, actually published by the camera driver. output should be remapped by the user.
+Services Called
+* `<camera>/request_image` ([polled_camera/GetPolledImage](https://github.com/ros-perception/image_common/blob/rolling/polled_camera/srv/GetPolledImage.srv))
+The camera driver's polled image service. camera should be remapped by the user to the camera namespace.