add timer

package.xml

@@ -2,12 +2,17 @@
   <name>robot_time</name>
   <version>0.7.10</version>
   <description>
-    robot_time is the second generation of the transform library, which lets
-    the user keep track of multiple coordinate frames over time. robot_time
-    maintains the relationship between coordinate frames in a tree
-    structure buffered in time, and lets the user transform points,
-    vectors, etc between any two coordinate frames at any desired
-    point in time.
+    robot_time provides time utilities for robot applications, similar to ros::Time
+    but independent of ROS. It includes:
+    - Time: Represents a point in time with nanosecond precision
+    - Duration: Represents a time interval (can be positive or negative)
+    - Rate: Helps run loops at a desired frequency (Hz)
+    - Timer: Calls a callback function periodically in a separate thread
+    - WallRate: Rate using wall-clock time instead of simulated time
+    
+    The library supports both simulated time and wall-clock time, and is designed
+    for use in control loops, event-driven systems, and multi-threaded applications.
+    All operations are thread-safe and cross-platform (Linux/Windows).
   </description>
   <author>Tully Foote</author>
   <author>Eitan Marder-Eppstein</author>