specify nightly version

README.md

@@ -13,5 +13,11 @@ libpso = { path = "path/to/libpso" }
 ```
 at the end of the `Cargo.toml` file
 
+# rust nightly version
+```
+rustup override set nightly-2021-03-15
+```
+
+
 # to actually run it
 * `cargo run`