docs: add a querying example into the main readme (#515)

* docs: add a querying example into the main readme

* Update README.rst

* Update README.rst

Co-authored-by: MF2199 <[email protected]>

Author: Ilya Gurov

README.rst

@@ -74,10 +74,8 @@ Format
         },
     }
 
-Example
-~~~~~~~
-
-For example:
+Database configurations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 .. code:: python
 
@@ -90,6 +88,25 @@ For example:
         }
     }
 
+Execute a query
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code:: python
+
+    from google.cloud.spanner_dbapi import connect
+
+    connection = connect("instance-id", "database-id")
+    cursor = connection.cursor()
+
+    cursor.execute(
+        "SELECT *"
+        "FROM Singers"
+        "WHERE SingerId = 15"
+    )
+
+    results = cur.fetchall()
+
+
 Limitations
 -----------