Add inheritance example

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+This example shows how to use persistent class inheritance.
+
+The example consists of the following files:
+
+employee.hxx
+  Header file defining the 'person' and 'employee' abstract persistent
+  classes as well as the 'permanent_employee', 'temporary_employee', and
+  'contractor' concrete persistent classes.
+
+employee-odb.hxx
+employee-odb.ixx
+employee-odb.cxx
+employee.sql
+  The first three files contain the database support code and the last file
+  contains the database schema for the employee.hxx header.
+
+  These files are generated by the ODB compiler from employee.hxx using the
+  following command line:
+
+  odb -d <database> --generate-schema --generate-query employee.hxx
+
+  Where <database> stands for the database system we are using, for example,
+  'mysql'.
+
+database.hxx
+  Contains the create_database() function which instantiates the concrete
+  database class corresponding to the database system we are using.
+
+driver.cxx
+  Driver for the example. It includes the employee.hxx and employee-odb.hxx
+  headers to gain access to the persistent classes and their database support
+  code. It also includes database.hxx for the create_database() function
+  declaration.
+
+  In main() the driver first calls create_database() to obtain the database
+  instance. It then persists a number of permanent and temporary employee
+  objects as well as a number of contractor objects. The next transaction
+  looks up a contractor based on the email address. Finally, the driver
+  performs a database query which uses a data member from the base class
+  in its criterion.
+
+To run the example we may first need to create the database schema (for some
+database systems, such as SQLite, the schema is embedded into the generated
+code which makes this step unnecessary). Using MySQL as an example, this
+can be achieved with the following command:
+
+mysql --user=odb_test --database=odb_test < employee.sql
+
+Here we use 'odb_test' as the database login and also 'odb_test' as the
+database name.
+
+Once the database schema is ready, we can run the example (using MySQL as
+the database):
+
+./driver --user odb_test --database odb_test