Addition of `readonly` option to ModelField to allow database generated fields

[Ben Cole]

There exist multiple use cases where a database will generate values for a field/column. Most RDBMSes expect that these columns be excluded from from INSERT and UPDATE statements. However they still need to be included in SELECT statements.

Examples include the PostgreSQL serial data type, triggers and functions that generate defaults on INSERT (such as PostgreSQL UUID)

The proposal therefore is to add a readonly option to the base Field class that when True would strip these fields from being compiled to SQL during INSERTs and UPDATEs. This allows for a very simple change that covers all possible write queries that Django may perform (including bulk_*).

There exists a proof of concept ​https://github.com/novafloss/django-readonly-field