Add info about constraint to zero-downtime section

[korycins]

Adding a constraint should be done in two steps:

• Adding unique index concurrently - example: https://github.com/saleor/saleor/pull/14991/files#diff-1707846f9ace0f20a69b72fba4b5ae2bad6f9d56bd648675d25d04f5044e69f5R15
• Converting the index into a constraint: example: https://github.com/saleor/saleor/pull/14948/files#diff-5fa6645ef508e60fa423588638ba52e6bc2ac6a08f9ec9fbb26defa6dae61d44R17

This should be documented in zero-downtime-section.