Improved git lfs instructions (#5707)

* Closes <#5526>
* [x] I have followed the instructions in the PR template

Author: lucasmerlin

CONTRIBUTING.md

@@ -35,13 +35,7 @@ You can test your code locally by running `./scripts/check.sh`.
 There are snapshots test that might need to be updated.
 Run the tests with `UPDATE_SNAPSHOTS=true cargo test --workspace --all-features` to update all of them.
 For more info about the tests see [egui_kittest](./crates/egui_kittest/README.md).
-
-We use [git-lfs](https://git-lfs.com/) to store big files in the repository.
-Make sure you have it installed (running `git lfs ls-files` from the repository root should list some files).
-Don't forget to run `git lfs install` after installing the git-lfs binary.
-You need to add any .png images to `git lfs`.
-If the CI complains about this, make sure you run `git add --renormalize .`.
-
+Snapshots and other big files are stored with git lfs. See [Working with lfs](#working-with-lfs) for more info.
 If you see an `InvalidSignature` error when running snapshot tests, it's probably a problem related to git-lfs.
 
 When you have something that works, open a draft PR. You may get some helpful feedback early!
@@ -51,6 +45,31 @@ Don't worry about having many small commits in the PR - they will be squashed to
 
 Please keep pull requests small and focused. The smaller it is, the more likely it is to get merged.
 
+## Working with git lfs
+
+We use [git-lfs](https://git-lfs.com/) to store big files in the repository.
+Make sure you have it installed (running `git lfs ls-files` from the repository root should list some files).
+Don't forget to run `git lfs install` in this repo after installing the git-lfs binary.
+You need to add any .png images to `git lfs` (see the .gitattributes file for rules and exclusions).
+If the CI complains about lfs, try running `git add --renormalize .`.
+
+Common git-lfs commands:
+```bash
+# Install git-lfs in the repo (installs git hooks)
+git lfs install
+
+# Move a file to git lfs
+git lfs track "path/to/file/or/pattern" # OR manually edit .gitattributes
+git add --renormalize . # Moves already added files to lfs (according to .gitattributes)
+
+# Move a file from lfs to regular git
+git lfs untrack "path/to/file/or/pattern" # OR manually edit .gitattributes
+git add --renormalize . # Moves already added files to regular git (according to .gitattributes)
+
+# Push to a contributor remote (see https://github.com/cli/cli/discussions/8794#discussioncomment-8695076)
+git push --no-verify
+```
+
 ## PR review
 
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