GitHub runs AsciiDoc documents it displays on the site (e.g., README.asciidoc) through the AsciiDoc processor with the header_footer attribute disabled. As a result, the h1 heading gets dropped from the rendered output.
You can retain the h1 heading by adding a [float] directive above the title, on the first line of the document.
Here's an example:
= Document title
Now the rendered output will include the h1 heading.