Update CONTRIBUTING.md to include updating subrepos (#2023)
Update CONTRIBUTING.md to include updating subrepos (#2023)

Signed-off-by: Will Martin <37d41699bdee4fcb969ca499eb0f8b82c60d59cc@engflow.com>
envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile