docs(sample): Add a top-level Datastore samples README (#790)

- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/java-datastore/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea. **Issue already existed**
- [x] Ensure the tests and linter pass **N/A**
- [x] Code coverage does not decrease (if any source code was changed) **N/A**
- [x] Appropriate docs were updated (if necessary) **Strictly a docs addition**

Fixes #745 ☕️

Note: I can confirm that the existing pom.xml works correctly.

Author: rwhogg

samples/README.md

@@ -0,0 +1,30 @@
+# Getting Started with Datastore Samples
+
+## Running a sample using Cloud Shell
+
+The Google Cloud Shell has application default credentials from its compute instance which will allow you to run samples without having to obtain `GOOGLE_APPLICATION_CREDENTIALS`.
+Go to the [Datastore Client Readme](https://github.com/googleapis/java-datastore#samples) to run each sample in the Cloud Shell.
+
+## Running a sample at the command line
+
+1. First set up the `GOOGLE_CLOUD_PROJECT` environment variable.
+
+2. Obtain authentication credentials.
+
+    Create local credentials by running the following command and following the
+    oauth2 flow (read more about the command [here][auth_command]):
+
+        gcloud auth application-default login
+
+    Or manually set the `GOOGLE_APPLICATION_CREDENTIALS` environment variable
+    to point to a service account key JSON file path.
+
+    Learn more at [Setting Up Authentication for Server to Server Production Applications][ADC].
+
+Now you can run a sample:
+
+1. `cd samples/snippets` - all samples are located in the `java-datastore/samples/snippets` directory.
+2. `mvn compile exec:java -Dexec.mainClass="com.google.datastore.snippets.TaskList"` - this runs the [TaskList sample](https://github.com/googleapis/java-datastore/blob/master/samples/snippets/src/main/java/com/google/datastore/snippets/TaskList.java).
+
+[ADC]: https://developers.google.com/identity/protocols/application-default-credentials
+[auth_command]: https://cloud.google.com/sdk/gcloud/reference/beta/auth/application-default/login