Create README.md with example usage instructions

Added instructions for running examples in README.

Author: vorburger

examples/README.md

@@ -0,0 +1,15 @@
+# Examples
+
+You can run these examples using:
+
+    git clone https://github.com/googleapis/java-genai.git
+    cd java-genai
+    mvn clean install -Djacoco.skip=true -DskipTests
+    
+    cd examples
+    export GOOGLE_API_KEY=...
+    mvn compile exec:java -Dexec.mainClass="com.google.genai.examples.GenerateContent"
+
+The `jacoco.skip` is required due to [issue #756](https://github.com/googleapis/java-genai/issues/756), and the `skipTests` just makes it faster.
+
+[Issue #801](https://github.com/googleapis/java-genai/issues/801) may avoid having to first `install` in the future.