Add contribution requirements to README.md
Add contribution requirements to README.md

Also `http://` -> `https://`
GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community