Added licenses with proper format and sources

[Antrikshgwal]

What does this PR do?

Add licences with sources

For resources

License Statement: Found on the book’s webpage, which specifies the license as CC BY-NC-SA Mastering Bitcoin - Unlocking Digital Currencies (2017), 2nd Edition:

License Statement: Found in the GitHub repository README, where it specifies CC BY-NC-ND. Competitive Programming Handbook by Antti Laaksonen:

License Statement: Found in the book’s Github, specifying the license as CC BY-NC-SA .

Description

Why is this valuable (or not)?

How do we know it's really free?

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

Checklist:

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

Follow-up

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

[github-actions[bot]]

Oh no 😟! Conflicts have been found.

Please 🙏, take a moment and address the merge conflicts of your pull request before we can evaluate it again.

Thanks in advance for your effort and patience ❤️!

[github-actions[bot]]

Linter failed, fix the error(s):

  free-programming-books/books/free-programming-books-subjects.md
  201:1-201:28  warning  Incorrect number of blank lines between heading and section  blank-lines-1-0-2  remark-lint