why #add_column doesn't support an :index option?

I’m curious how come the #add_column method in migrations does not support an optional :index option, which would also create an index for that column.

Since there is the possibility to do something similar like:

create_table :taggings do |t  |
t.references :tagger, polymorphic: true, index: true

I’m curious as to if there’s any practical reason due to which this feature isn’t implemented or if it’s just that nobody thought of it yet. If it’s feasible I couldl start working on it and submit a PR.