HashMap Data Structure: Add link to pure function explaination

[phucfix]

Add link to pure function explanation in README

Because

Readers may not be familiar with the term "pure function," so adding an external reference helps improve clarity and understanding of the documentation.

This PR

• Add a link in the README to an explanation of pure functions

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[mao-sz]

Thanks. The same change needs to be made to the Ruby course's hash map lesson. The change can be made in this same PR - no need to open a new PR.