The GitHub markdown code:
1. First item
* subitem
1. Second item
Generates a big space between the First
, Second
and
You cannot do this going from a primary to secondary item. It is possible from a secondary to tertiary list, however. This is due to GitHub's styling of the markdown, not an inherent issue with the markdown itself.
See this document for details on what is possible regarding different layout styles.
An admittably "hacky" but still working solution:
1. First item [< two spaces for the linebreak]
◦ subitem1 [< two spaces for the linebreak]
◦ subitem2
2. Second item
You need to indent the sublist with at least 4 spaces:
1. First item
* subitem
2. Second item