Document local testing instructions (#1672)

<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:

- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->

## Summary

Provides an example of local testing usage, to make it easier for new
developers who are less familiar with `cargo` / `rust` development best
practices.

## Test Plan

I followed the setup instructions earlier in the contributing guide,
then ran these 2 commands.

CONTRIBUTING.md

@@ -30,7 +30,7 @@ See the [Python](#python) section for instructions on installing the Python vers
 
 ### Windows
 
-You can install CMake from the [installers](https://cmake.org/download/) or with `pipx install cmake` 
+You can install CMake from the [installers](https://cmake.org/download/) or with `pipx install cmake`
 (make sure that the pipx install path is in `PATH`, pipx complains if it isn't).
 
 ## Testing
@@ -52,6 +52,15 @@ Alternatively, you can install `zstandard` from PyPI, then run:
 python3.12 scripts/bootstrap/install.py
 ```
 
+### Local testing
+
+You can invoke your development version of uv with `cargo run -- <args>`. For example:
+
+```shell
+cargo run -- venv
+cargo run -- pip install requests
+```
+
 ## Running inside a docker container
 
 Source distributions can run arbitrary code on build and can make unwanted modifications to your system (["Someone's Been Messing With My Subnormals!" on Blogspot](https://moyix.blogspot.com/2022/09/someones-been-messing-with-my-subnormals.html), ["nvidia-pyindex" on PyPI](https://pypi.org/project/nvidia-pyindex/)), which can even occur when just resolving requirements. To prevent this, there's a Docker container you can run commands in: