normalize readme
normalize readme

* simplified wording
* Pivotal's Cloud Foundry -> Pivotal Cloud Foundry
* Headings to Sentence case
* Be more open to changes/PRs
* License badge
GoogleCloudPlatform/service-broker-samples,GoogleCloudPlatform/service-broker-samples,GoogleCloudPlatform/service-broker-samples,GoogleCloudPlatform/service-broker-samples