added codecov badge to cargo.toml
diff --git a/Cargo.toml b/Cargo.toml
index bf85e91..436ef7a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,6 +14,7 @@
 [badges]
 travis-ci = { repository = "heartsucker/rust-tuf", branch = "master" }
 appveyor = { repository = "heartsucker/rust-tuf", branch = "master", service = "github" }
+codecov = { repository = "heartsucker/rust-tuf", branch = "master", service = "github" }
 
 [lib]
 name = "tuf"