docs: Update samples' README.md to ensure given (#3420)

commands run samples.

Author: gagangupt16

samples/README.md

@@ -18,29 +18,23 @@ Install [Maven](http://maven.apache.org/).
 Build your project from the root directory (`java-spanner`):
 
     mvn clean package -DskipTests
+    cd samples/snippets
+    mvn package
 
-Every subsequent command here should be run from a subdirectory (`cd samples/snippets`).
+Every subsequent command here should be run from a subdirectory `samples/snippets`.
 
-You can run a given `ClassName` via:
+### Running samples
 
-    mvn exec:java -Dexec.mainClass=com.example.spanner.ClassName \
-        -DpropertyName=propertyValue \
-        -Dexec.args="any arguments to the app"
+Usage:
 
-### Running a simple query (using the quickstart sample)
+    java -jar target/spanner-snippets/spanner-google-cloud-samples.jar operation my-instance my-database
 
-    mvn exec:java -Dexec.mainClass=com.example.spanner.QuickstartSample -Dexec.args="my-instance my-database"
+#### Examples
 
-## Tutorial
+Create Database:
 
-### Running the tutorial
-    mvn exec:java -Dexec.mainClass=com.example.spanner.admin.archived.SpannerSample -Dexec.args="<command> my-instance my-database"
+    java -jar target/spanner-google-cloud-samples-jar-with-dependencies.jar my-instance my-database
 
-## Tracing sample
-`TracingSample.java` demonstrates how to export traces generated by client library to StackDriver and to /tracez page.
+Listing database operations:
 
-### Running the tracing sample
-    mvn exec:java -Dexec.mainClass=com.example.spanner.TracingSample -Dexec.args="my-instance my-database"
-
-## Test
-    mvn verify -Dspanner.test.instance=<instance id> -Dspanner.sample.database=<new database id>  -Dspanner.quickstart.database=<existing database id>
+    java -jar target/spanner-snippets/spanner-google-cloud-samples.jar listdatabaseoperations my-instance my-database