hydra.sql: embed some in-line docs about schema changes

Dependencies

• [2] SB2V735V Keep track of the database schema version
• [*] N22GPKYT * Put info about logs / build products in the DB.

Change contents

• edit in src/sql/hydra.sql at line 1
  [4.1]

  [2.32665]
  + -- Making a database change:

  + --

  + -- 1. Update this schema document to match what the end result should be.

  + --

  + -- 2. Run `make -C src/sql update-dbix hydra-postgresql.sql` in the root

  + -- of the project directory, and git add / git commit the changed,

  + -- generated files.

  + --

  + -- 3. Create a migration in this same directory, named `upgrade-N.sql`

  + --

  +