Fix broken link in readme and point to documentation (#960)
Fix broken link in readme and point to documentation (#960)

Redoing fix done in #959 as repo submitting the PR is deleted before merged in.
Let this link point to our documentation instead.
GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp