Add contribution instructions

Change-Id: Icc58ef6c1e79e59977e0093973ad353237560e8e

CONTRIBUTING.md

@@ -19,7 +19,44 @@ again.
 
 All submissions, including submissions by project members, require review. We
 use a [Gerrit](https://www.gerritcodereview.com) instance hosted at
-https://chromium-review.googlesource.com for this purpose. See the
+https://chromium-review.googlesource.com for this purpose.
+
+## Sending patches
+
+The basic git workflow for modifying libwebp code and sending for review is:
+
+1.  Get the latest version of the repository locally:
+
+    ```sh
+    git clone https://chromium.googlesource.com/webm/libwebp && cd libwebp
+    ```
+
+2.  Copy the commit-msg script into ./git/hooks (this will add an ID to all of
+    your commits):
+
+    ```sh
+    curl -Lo .git/hooks/commit-msg https://chromium-review.googlesource.com/tools/hooks/commit-msg && chmod u+x .git/hooks/commit-msg
+    ```
+
+3.  Modify the local copy of libwebp. Make sure the code
+    [builds successfully](https://chromium.googlesource.com/webm/libwebp/+/HEAD/doc/building.md#cmake).
+
+4.  Choose a short and representative commit message:
+
+    ```sh
+    git commit -a -m "Set commit message here"
+    ```
+
+5.  Send the patch for review:
+
+    ```sh
+    git push https://chromium-review.googlesource.com/webm/libwebp HEAD:refs/for/main
+    ```
+
+    Go to https://chromium-review.googlesource.com to view your patch and
+    request a review from the maintainers.
+
+See the
 [WebM Project page](https://www.webmproject.org/code/contribute/submitting-patches/)
 for additional details.