Add database connection string default value (#186)

Author: Malax

src/main/resources/application.properties

@@ -1,2 +1,6 @@
 server.port=${PORT:5000}
-spring.datasource.url=${JDBC_DATABASE_URL}
+# Default to "jdbc:postgresql://example.com:5432/database" so that the application at least starts up when
+# JDBC_DATABASE_URL is not set. We use this here to reduce friction when newcomers work with this getting started
+# application. Production applications should not have a default like this, especially not ones that have credentials
+# in them!
+spring.datasource.url=${JDBC_DATABASE_URL:jdbc:postgresql://example.com:5432/database}