Add another sentence to the Description about the capabilities

Author: DavisVaughan

DESCRIPTION

@@ -16,7 +16,9 @@ Description: Provides a comprehensive library for date-time
     manipulations using a new family of orthogonal date-time classes
     (durations, time points, zoned-times, and calendars) that partition
     responsibilities so that the complexities of time zones are only
-    considered when they are really needed.
+    considered when they are really needed. Capabilities include: date-time
+    parsing, formatting, arithmetic, extraction and updating of components,
+    and rounding.
 License: MIT + file LICENSE
 URL: https://github.com/r-lib/clock
 BugReports: https://github.com/r-lib/clock/issues