[ty] (docs) - Note insta is required for working with ty tests in ty CONTRIBUTING.md (#22332)

crates/ty/CONTRIBUTING.md

@@ -25,6 +25,12 @@ that are ready for contributions.
 ty is written in Rust. You'll need to install the
 [Rust toolchain](https://www.rust-lang.org/tools/install) for development.
 
+You'll also need [Insta](https://insta.rs/docs/) to update snapshot tests:
+
+```shell
+cargo install cargo-insta
+```
+
 You'll need [uv](https://docs.astral.sh/uv/getting-started/installation/) (or `pipx` and `pip`) to
 run Python utility commands.
 
@@ -68,6 +74,13 @@ will save you time and expedite the merge process.
 
 If you're using VS Code, you can also install the recommended [rust-analyzer](https://marketplace.visualstudio.com/items?itemName=rust-lang.rust-analyzer) extension to get these checks while editing.
 
+Note that many code changes also require updating the snapshot tests, which is done interactively
+after running `cargo test` like so:
+
+```shell
+cargo insta review
+```
+
 Include the text `[ty]` at the beginning of your pull request title, to distinguish ty pull requests
 from Ruff ones.