Add an explicit distinction between a command's description and summary.

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 47c0cb5aa1d0a874daab0ce3f619269d6772f575..ae90612d5dea8ed89c64babf2180e9f6acdff870 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,14 @@
+## 0.13.4
+
+* By default, only the first line of a command's description is included in its
+  parent runner's usage string. This returns to the default behavior from
+  before 0.13.3+1.
+
+* A `Command.summary` getter has been added to explicitly control the summary
+  that appears in the parent runner's usage string. This getter defaults to the
+  first line of the description, but can be overridden if the user wants a
+  multi-line summary.
+
 ## 0.13.3+6
 
 * README fixes.