There's an unnecessary top padding between the header and the first item shown in this picture.
How it can be removed?
you can find the source code here: https://github.com/chrisbanes/cheesesquare
You can override predefined dimensions at your dimens.xml
as;
<dimen name="design_navigation_padding_top_default" tools:override="true">0dp</dimen>
<dimen name="design_navigation_separator_vertical_padding" tools:override="true">0dp</dimen>
<dimen name="design_navigation_padding_bottom" tools:override="true">0dp</dimen>
Other possible values are here: https://github.com/android/platform_frameworks_support/blob/master/design/res/values/dimens.xml