Add Showcases Services to README.md. Resolves #5
Add Showcases Services to README.md. Resolves #5

Initially I created a markdown table, but I decided that over time a
table would be too difficult to read.

GoogleCloudPlatform/service-broker-samples,GoogleCloudPlatform/service-broker-samples,GoogleCloudPlatform/service-broker-samples,GoogleCloudPlatform/service-broker-samples