Add a ReadMe

README.md

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+# MSCKF
+
+Experimental implementation of the Multi-State Constraint Kalman Filter with ROS interface.
+
+Based on the paper by Mourikis and Roumeliotis ([PDF](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.125.2169&rep=rep1&type=pdf)).
+
+Also includes a feature tracking node using OpenCV's SIFT implementation.
+
+This is a work in progress. In particular, The API is a bit non-standard and is subject to change.
+
+## ROS nodes
+
+### feature_tracking_node
+#### Subscribed topics
+
+- `camera/image_rect` (sensor_msgs/Image) - Undistorted image stream.
+- `camera/camera_info` (sensor_msgs/CameraInfo) - The corresponding camera metadata.
+
+
+#### Published topics
+
+- `output_image` (sensor_msgs/Image) - Image with arrows drawn on it, showing matches.
+- `features` (msckf/ImageFeatures) - custom message describing feature positions
+
+### msckf_node
+#### Subscribed topics
+- `imu_vel` (geometry_msgs/TwistStamped) - IMU measurement of linear and angular velocity.
+- `odom` (nav_msgs/Odometry) - Position of robot, used to initialize the filter
+- `camera/image_rect` (sensor_msgs/Image)-
+the same image stream used by `feature_tracking_node`. Currently, only the header information is used.
+- `features` (msckf/ImageFeatures) - the feature information published by `feature_tracking_node`
+
+#### Published topics
+- `odom_combined` (geometry_msgs/PoseWithCovarianceStamped) - Estimated robot pose