Update QUICHE contributing guidelines

PiperOrigin-RevId: 255091609
Change-Id: I7c4cececc4895124e3f49b74c985f80b056fdf3e

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 3a93363..07824e0 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -17,10 +17,13 @@
 
 ## Code reviews
 
-All submissions, including submissions by project members, require review. We
-use Gerrit pull requests for this purpose.
+The QUICHE repository is currently not set up to accept pull requests directly.
+If you would like to make a contribution, please follow these steps:
 
-TODO: write up the contributing guidelines.
+1. Sign the Contributor License Agreement (see above).
+2. Create a Gerrit pull request at <https://quiche-review.googlesource.com>.
+3. Email a link to your pull request to <quiche-contribution@google.com>.
+4. An engineer will review your pull request and merge it internally.
 
 ## Community Guidelines