chore: add instructions to fix lfs errors (#34732)

### Reason for this change

A new Github action to check for LFS files was added and users without LFS cannot have their PR pass this test if they updated an integ test.

### Description of changes

Add instructions on how to check if git lfs is installed, how to install it and fix the problematic files.

### Description of how you validated changes

Helped a contributor make the test pass on his PR.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: leonmk-aws

CONTRIBUTING.md

@@ -1465,6 +1465,17 @@ have to disable the built-in rebuild functionality of `lerna run test`:
 $ CDK_TEST_BUILD=false lr test
 ```
 
+#### The check-lfs Github action fails on my PR
+
+This happens if your PR has files that should have been a Git LFS pointer but were not. You should verify
+that you have Git LFS installed with `git lfs`, if not you can install it with `git lfs install`.
+Once Git LFS is installed, use the following command:
+
+```shell
+$ git lfs migrate import --no-rewrite <path to files that make the gh action fail>
+```
+This will create a new commit that you can push to your branch to make the Github action pass.
+
 ## Debugging
 
 ### Connecting the VS Code Debugger