Copy description from Cargo.toml to METADATA

Test: make
Change-Id: I5aac9c603e87d32c05f9a7cca51bfcd12c5ae7c2
diff --git a/METADATA b/METADATA
index 059a608..a0eb879 100644
--- a/METADATA
+++ b/METADATA
@@ -1,5 +1,5 @@
 name: "async-trait"
-description: "Async trait methods."
+description: "Type erasure for async trait methods"
 third_party {
   url {
     type: HOMEPAGE