fix(flutter/flutter#192640): Add git lfs instructions

CONTRIBUTING.md

@@ -32,6 +32,75 @@ Before you get started, we encourage you to read these documents which describe
 2. [Values](./docs/about/Values.md),
    which talks about what we care most about.
 
+Media and Git LFS
+-----------------
+
+Use [Mermaid](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams) (`mermaid` code blocks) or ASCII art for architecture, sequence, and state diagrams. When those formats are insufficient, prefer non-binary `.svg` graphics over binary images, and use binary images only for rendered screenshots. Store all graphics and screenshots in `rfc/media/`.
+
+Plain-text `.svg` files are tracked directly by Git. Binary media files (`.png`, `.jpg`, `.gif`, `.webp`, `.mp4`, `.pdf`, and others listed in [`.gitattributes`](.gitattributes)) must be stored with Git Large File Storage (Git LFS). CI blocks pull requests that commit raw binary blobs instead of LFS pointers.
+
+### 1. Install Git LFS
+
+Install the `git-lfs` client for your operating system, then run `git lfs install` once per user account to register Git's smudge and clean filters.
+
+#### macOS
+
+```sh
+brew install git-lfs
+git lfs install
+```
+
+#### Windows
+
+Git LFS ships by default with [Git for Windows](https://gitforwindows.org/). If it is not installed, install it with WinGet and restart your terminal so `PATH` updates:
+
+```powershell
+winget install --id GitHub.GitLFS -e
+git lfs install
+```
+
+#### Linux
+
+```sh
+# Debian / Ubuntu
+sudo apt update && sudo apt install git-lfs
+
+# Fedora / RHEL
+sudo dnf install git-lfs
+
+# Arch Linux
+sudo pacman -S git-lfs
+
+git lfs install
+```
+
+Homebrew on Linux (`brew install git-lfs`) is also supported. Verify the installation with `git lfs version`. If you cloned this repository before installing Git LFS, run `git lfs pull` to download existing media assets.
+
+### 2. Add screenshots to an RFC
+
+[`.gitattributes`](.gitattributes) already configures LFS patterns for this repository, so you do not need to run `git lfs track`.
+
+1. Save screenshots in `rfc/media/` using lowercase kebab-case names (for example, `rfc/media/impeller-glyph-atlas.png`).
+2. Reference them with a relative path from your RFC in `rfc/` (for example, `![Glyph atlas debug view](media/impeller-glyph-atlas.png)`).
+3. Stage and commit as usual, then verify LFS pointer tracking locally with [`bin/lfs_verify.dart`](bin/lfs_verify.dart):
+
+```sh
+git add rfc/media/impeller-glyph-atlas.png
+git lfs ls-files
+dart pub get
+dart run bin/lfs_verify.dart --base-branch main
+```
+
+### 3. Fix files committed without Git LFS
+
+If you committed a binary file before running `git lfs install`, CI (`bin/lfs_verify.dart`) will fail. Because this repository squash-merges pull requests, you can fix the branch tip without rewriting history:
+
+```sh
+git lfs install
+git add --renormalize .
+git commit -m "chore: serialize media files with Git LFS"
+```
+
 Helping out in the issue database
 ---------------------------------
 
@@ -241,4 +310,4 @@ Social events in the contributor community
 Finally, one area where you could have a lot of impact is in contributing to social interactions among the Flutter contributor community itself.
 This could take the form of organizing weekly video chats on our Discord, or planning tech talks from contributors, for example.
 If this is an area that is of interest to you, please join our [Discord](./docs/contributing/Chat.md) and ping Hixie on the #hackers
-channel!
+channel!