Updated Backtracking readme

[Akshat-Pandey16]

Updated the readme with reference links for pre-existing codes in the repository.

Describe your change:

• [ ] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [x] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [ ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ ] All new Python files are placed inside an existing directory.
• [ ] All filenames are in all lowercase characters with no spaces or dashes.
• [ ] All functions and variable names follow Python naming conventions.
• [ ] All function parameters and return values are annotated with Python type hints.
• [ ] All functions have doctests that pass the automated testing.
• [ ] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [ ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.