Update Cargo.toml

Add maintenance badge [ci skip]
diff --git a/Cargo.toml b/Cargo.toml
index 9b940d8..9346f56 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,6 +13,7 @@
 [badges]
 travis-ci = { repository = "kkawakam/rustyline" }
 appveyor = { repository = "kkawakam/rustyline" }
+maintenance = { status = "actively-developed" }
 
 [dependencies]
 libc = "0.2"