Add a style guide for databases

[ThierryO]

Should, if possible, accommodate different kinds of databases.

Scope:

• PostgreSQL
• SQL Server
• SQLite?
• Access?
• MySQL?

[florisvdh]

The pull request is at #140 . To be discussed further.

[florisvdh]

@fredericpiesschaert what's the relation between #140 and #169 ?

[fredericpiesschaert]

@florisvdh there is no relation, #169 is about writing easy to read sql-scripts, #140 about designing databases

[florisvdh]

Cool @fredericpiesschaert , I've clarified this in the PR title.