demonstrate alterTable and addColumn in example migration

[peterbecich]

This PR improves the V0002 example migration to include a demo of alterTable and addColumn.

The example already demonstrated the preserve function but not these two other functions.

Now the V0002 migration additionally adds a salary field to Staff.

Here is the addition to the migration, rendered to SQL:

SQL command type: PgCommandTypeDdl
SQL script: 
ALTER TABLE "staff" ADD COLUMN "salary" SMALLINT DEFAULT 100 NOT NULL

Printing this depends on #732

[LaurentRDC]

Good idea, thanks!