Document that JSONField requires Psycopg 2.5.4+

[Kevin Campbell]

When using Django 1.9 with psycopg2==2.5.2 and JSONField for postgres, the field will be correctly stored as jsonb in the postgres database, but will be converted to a string when read back.

from django.contrib.postgres.fields import JSONField
class TestModel(models.Model):
    data = JSONField(null=True)

tm = TestModel(data=[1,2,3])
tm.save()

select * from bugtest_testmodel
 id |   data
----+-----------
  1 | [1, 2, 3]

select data->0 from bugtest_testmodel;
 ?column?
----------
 1

tm = TestModel.objects.all()[0]
tm.data
'[1, 2, 3]'

The issue is resolved in the latest psycopg2 version (2.6.1). It's somewhat debatable if this is a bug, but as the behaviour does not match the Django manual I think it would be best to raise a Warning when using JSONField with older versions of psycopg2.

[Tim Graham]

Psycopg 2.5.4 added jsonb support. I think it's probably enough to document this.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In b26d147:

Fixed #25949 -- Documented Psycopg2 version requirement for JSONField.

[Tim Graham <timograham@…>]

In 571f7b75:

[1.9.x] Fixed #25949 -- Documented Psycopg2 version requirement for JSONField.

Backport of b26d147259202455ce356755b729c3dcb16ae9e6 from master