Append rows to a pandas DataFrame without making a new copy

Append rows to a pandas DataFrame without making a new copy

The pandas documentation includes a note:
Note Unlike list.append method, which appends to the original list and
returns nothing, append here does not modify df1 and returns its copy with
df2 appended.
How can I append to an existing DataFrame without making a copy? Or in the
terms of the note how can I modify df1 in place by appending df2 and
return nothing?