docs(samples): add readme (#849)

* docs(samples): add readme

* address reviewer comments

Author: anguillanneuf

samples/README.md

@@ -0,0 +1,25 @@
+# How to run the samples
+
+1. Clone this repo and navigate to the `snippets` directory.
+   ```shell
+   git clone https://github.com/googlepis/java-pubsub
+   cd java-pubsub/samples/snippets
+   ```
+
+1. [Obtain GCP user access credentials] via a web flow for your application. 
+   ```shell
+   gcloud auth application-default login
+   ```
+
+1. With [Maven] installed, run a sample that has the to-do section completed:
+   ```shell
+   mvn clean compile exec:java -Dexec.mainClass=pubsub.CreateTopicExample
+   ```
+   
+1. To run the samples test:
+   ```shell
+   mvn -Dtest=pubsub.AdminIT test
+   ```
+
+[Maven]: https://maven.apache.org/
+[Obtain GCP user access credentials]: https://cloud.google.com/sdk/gcloud/reference/auth/application-default/login