Clarify CHANGELOG: TTL indexes must be on Time-ish field

As per the linked mongodb docs:

* the indexed field must be a date BSON type. If the field does not have a date type, the data will not expire.
* you cannot create this index on the _id field, or a field that already has an index.
* the TTL index may not be compound (may not have multiple fields).
* if the field holds an array, and there are multiple date-typed data in the index, the document will expire when the lowest (i.e. earliest) matches the expiration threshold.
* you cannot use a TTL index on a capped collection, because MongoDB cannot remove documents from a capped collection.
Merge pull request #2833 from Nielsomat/patch-2

Clarify CHANGELOG: TTL indexes must be on Time-ish field
Each validator must determine if it needs the value before type cast or not.
Update changelog
#store_in called multiple times merges options instead of replacing them
Update changelog
Eager load application when indexing.

[ fix #2807 ]
Add code climate to readme
model.touch when using short ts

Making touch method update the updated_at field, even when using alias
Update changelog
remove !_assigning? check on batch_remove

Remove the _assigning check, so we can destroy embedded docs inside
a setter. [ #2841 ]
Add spec, update changelog
Fix no_timeout option handling.

[ #2840 ]
Set klass to document klass when nil. [ fix #2774 ]
Reapply extensions on relations in Marshal.load.

[ fix #2822 ]
Fixing JRuby DateTime usec precision
Fix BigDecimal NaN demongoization.

[ close #2851 ]
Adding specs for #2843
Mongoize dynamic fields.

[ fix #2826 ]
Fix options clear in uniqueness validator. [ fix #2835 ]