docs: add simple connection sample to readme (#263)

Adds a simple code example for connecting to Cloud Spanner using JDBC.
Also fixes a wrong link to the online documentation.

README.md

@@ -58,8 +58,30 @@ See the [Google Cloud Spanner JDBC client library docs][javadocs] to learn how t
 use this Google Cloud Spanner JDBC Client Library.
 
 
-
-
+### Creating a JDBC Connection
+
+The following example shows how to create a JDBC connection to Cloud Spanner and execute a simple query.
+
+```java
+String projectId = "my-project";
+String instanceId = "my-instance";
+String databaseId = "my-database";
+
+try (Connection connection =
+    DriverManager.getConnection(
+        String.format(
+            "jdbc:cloudspanner:/projects/%s/instances/%s/databases/%s",
+            projectId, instanceId, databaseId))) {
+  try (Statement statement = connection.createStatement()) {
+    try (ResultSet rs = statement.executeQuery("SELECT CURRENT_TIMESTAMP()")) {
+      while (rs.next()) {
+        System.out.printf(
+            "Connected to Cloud Spanner at [%s]%n", rs.getTimestamp(1).toString());
+      }
+    }
+  }
+}
+```
 
 
 ## Troubleshooting
@@ -101,7 +123,7 @@ Java 8 OSX | [![Kokoro CI][kokoro-badge-image-3]][kokoro-badge-link-3]
 Java 8 Windows | [![Kokoro CI][kokoro-badge-image-4]][kokoro-badge-link-4]
 Java 11 | [![Kokoro CI][kokoro-badge-image-5]][kokoro-badge-link-5]
 
-[product-docs]: https://cloud.google.com/pubsub/docs/
+[product-docs]: https://cloud.google.com/spanner/docs/use-oss-jdbc
 [javadocs]: https://googleapis.dev/java/google-cloud-spanner-jdbc/latest/index.html
 [kokoro-badge-image-1]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-spanner-jdbc/java7.svg
 [kokoro-badge-link-1]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-spanner-jdbc/java7.html