How to make heading collapsible in Readme markdown file for GitHub?

You can't. For security reasons, GitHub will strip out any code which would accomplish what you want.

As a reminder, Markdown simply renders to HTML. So the answer to your question would be the same as it would for HTML. You would need some CSS and/or JavaScript to control the accordion-like behavior (see multiple examples here). However, for security reasons, GitHub does not allow CSS or JavaScript from users on its site and therefore they strip all CSS and JavaScript out after rendering the Markdown to HTML.

GitHub documents how they process user supplied markup in github/markup. As described in step 1 there, your Markdown is converted to HTML. However, in step 2:

2. The HTML is sanitized, aggressively removing things that could harm you and your kin—such as script tags, inline-styles, and class or id attributes.

That being the case, there is no way to provide the necessary JavaScript and/or CSS needed to accomplish what you want.

As an alternative, you may want to consider hosting your documents on a site which does not provide such restrictions.

You can use this feature with headings: Organizing information with collapsed sections - GitHub Docs

e.g.

<details>
<summary>
Installation
</summary>

## Installation
</details>

Becomes:

which opens to: