How do I add a placeholder on a CharField in Django?

Take this very simple form for example:

class SearchForm(Form):
    q = forms.CharField(label="search")

This gets rendered in the template:

<input type="text" name="q" id="id_q" />

However, I want to add the placeholder attribute to this field with a value of Search so that the HTML would look something like:

<input type="text" name="q" id="id_q" placeholder="Search" />

Preferably I would like to pass the placeholder value in to CharField in the form class through a dictionary or something like:

q = forms.CharField(label="search", placeholder="Search")

What would be the best way to accomplish this?