add include directive (#450)

Author: Byron

git-refspec/Cargo.toml

@@ -6,6 +6,7 @@ license = "MIT/Apache-2.0"
 description = "A WIP crate of the gitoxide project for parsing and representing refspecs"
 authors = ["Sebastian Thiel <[email protected]>"]
 edition = "2018"
+include = ["src/**/*", "CHANGELOG.md", "README.md"]
 
 [lib]
 doctest = false