Remove README advice on sentinel values

I think this advice is misleading for most datasets and most RDBMS this
is not necessary, and as per #206 doesn't even work.

I have moved it to the wiki
https://github.com/radar/paranoia/wiki/Custom-sentinel-values

README.md

@@ -184,27 +184,6 @@ You can replace the older `acts_as_paranoid` methods as follows:
 The `recover` method in `acts_as_paranoid` runs `update` callbacks.  Paranoia's
 `restore` method does not do this.
 
-## Support for Unique Keys with Null Values
-
-Most databases ignore null columns when it comes to resolving unique index
-constraints.  This means unique constraints that involve nullable columns may be
-problematic. Instead of using `NULL` to represent a not-deleted row, you can pick
-a value that you want paranoia to mean not deleted. Note that you can/should
-now apply a `NOT NULL` constraint to your `deleted_at` column.
-
-Per model:
-
-```ruby
-# pick some value
-acts_as_paranoid sentinel_value: DateTime.new(0)
-```
-
-or globally in a rails initializer, e.g. `config/initializer/paranoia.rb`
-
-```ruby
-Paranoia.default_sentinel_value = DateTime.new(0)
-```
-
 ## License
 
 This gem is released under the MIT license.