Add Cargo.toml badges for published packages.

This adds a basic travis badge, as well as a badge indicating
"experimental" status, since the APIs are still evolving.

lib/cretonne/Cargo.toml

@@ -18,3 +18,7 @@ name = "cretonne"
 # Please don't add any unless they are essential to the task of creating binary
 # machine code. Integration tests that need external dependencies can be
 # accomodated in `tests`.
+
+[badges]
+maintenance = { status = "experimental" }
+travis-ci = { repository = "Cretonne/cretonne" }