Add "include" to Cargo.toml files

This filters out junk files from the crates.

Closes #331.

serde_macros/Cargo.toml

@@ -7,6 +7,7 @@ description = "Macros to auto-generate implementations for the serde framework"
 repository = "https://github.com/serde-rs/serde"
 documentation = "https://github.com/serde-rs/serde"
 keywords = ["serde", "serialization"]
+include = ["Cargo.toml", "src/**/*.rs"]
 
 [lib]
 name = "serde_macros"