Update contributing guide to mention snapshot test updating (#8185)

### Description

Had to ask this info from the rest of the team, ensures it's documented.

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### Testing Instructions

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CONTRIBUTING.md

@@ -6,6 +6,7 @@ Thanks for your interest in contributing to Turbo!
 
 - [Contributing to Turbo](#contributing-to-turbo)
   - [General Dependencies](#general-dependencies)
+    - [Linux Dependencies](#linux-dependencies)
   - [Contributing to Turborepo](#contributing-to-turborepo)
     - [Building Turborepo](#building-turborepo)
     - [TLS Implementation](#tls-implementation)
@@ -290,6 +291,14 @@ You can also create a little demo app and run
 cargo run -p node-file-trace -- print demo/index.js
 ```
 
+Updating snapshot tests:
+
+In case of changes that involve updating snapshots you can use the `UPDATE=1` environment variable.
+
+```
+UPDATE=1 cargo nextest run
+```
+
 ### Profiling Turbopack
 
 See [the profiling docs for Turbopack](https://turbo.build/pack/docs/advanced/profiling) for details.