Put it in the source/_includes/custom/asides directory and then adapt your
_config.yml accordingly. As example, here is how it is done on this site:
# ...# list each of the sidebar modules you want to include, in the order you want them to appear.# To add custom asides, create files in /source/_includes/custom/asides/ and add them to the list like 'custom/asides/custom_aside_name.html'default_asides:[asides/recent_posts.html,custom/asides/category_list.html]# Each layout uses the default asides, but they can have their own asides instead. Simply uncomment the lines below# and add an array with the asides you want to use.blog_index_asides:[asides/what_is_octostrap3.html,asides/recent_posts.html,custom/asides/category_list.html,asides/github.html]# post_asides:# page_asides:# ...