Add deprecation note to readme (#46)

* add deprecation note to readme

Author: KodrAus

README.md

@@ -9,6 +9,10 @@ contents when the directory is dropped.
 
 [Documentation](https://doc.rust-lang.org/tempdir)
 
+## Deprecation Note
+
+The `tempdir` crate is being merged into [`tempfile`](https://github.com/Stebalien/tempfile). Please see [this issue](https://github.com/Stebalien/tempfile/issues/43) to track progress and direct new issues and pull requests to `tempfile`.
+
 ## Usage
 
 Add this to your `Cargo.toml`: