Update release.md to reflect notes taken during 3.5.15 release

Update release.md based on 3.5.15 release: #18247 (comment)

Signed-off-by: Wenjia Zhang <wenjiazhang@google.com>

Documentation/contributor-guide/release.md

@@ -62,6 +62,7 @@ which don't need to be executed before releasing each version.
 4. Authenticate the image registry, refer to [Authentication methods](https://cloud.google.com/container-registry/docs/advanced-authentication).
    - `gcloud auth login`
    - `gcloud auth configure-docker`
+5. Ensure there is at least 500MB of free space on your Linux machine.
 
 ### Release steps
 
@@ -79,13 +80,14 @@ which don't need to be executed before releasing each version.
    under project `etcd-development`, and images are pushed to `quay.io` and `gcr.io`.
 5. Publish the release page on GitHub
    - Set the release title as the version name
+   - Choose the correct release tag (generated from step #4)
    - Follow the format of previous release pages
    - Attach the generated binaries and signature file
    - Select whether it's a pre-release
    - Publish the release
 6. Announce to the etcd-dev googlegroup
 
-   Follow the format of previous release emails sent to etcd-dev@googlegroups.com, see an example below,
+   Follow the format of previous release emails sent to etcd-dev@googlegroups.com, see an example below. After sending out the email, ask one of the mailing list maintainers to approve the email from the pending list.
 ```
 Hello,