add: "Design Patterns Elements of Reusable Object-Oriented Software" by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides

[arya2004]

What does this PR do?

Add resource(s)

For resources

Description

This book goes beyond introduction to object-oriented technology. It describes simple and elegant solutions to specific problems in object-oriented software design

Why is this valuable (or not)?

It's the book recommended by most university courses on Object-Oriented Programming

How do we know it's really free?

A direct download link is available to everyone.

For book lists, is it a book? For course lists, is it a course? etc.

It is a book.

Checklist:

• [x] Read our contributing guidelines.
• [x] Search for duplicates.
• [x] Include author(s) and platform where appropriate.
• [x] Put lists in alphabetical order, correct spacing.
• [x] Add needed indications (PDF, access notes, under construction).
• [x] Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!

[eshellman]

This appears not to be an authorized distribution. Please see docs/CONTRIBUTING.md guidelines.