Added ISL course recommendation

[Smcgb]

This pull request proposes the addition of the "Introduction to Statistical Learning" course to the Open Source Society University (OSSU) Data Science curriculum. This course provides a comprehensive introduction to statistical learning methods and is essential for students pursuing a career in data science.

Rationale:

Relevance to Data Science: Statistical learning is a fundamental aspect of data science, providing the necessary tools for data analysis and interpretation.

Complement Existing Curriculum: This course fills gaps in the current curriculum by offering a focused study on statistical learning techniques.

Quality of Instruction: The course is taught by renowned experts in the field and is well-received by learners for its clarity and depth. Anyone who wants to intelligently analyze complex data should own this book.” Larry Wasserman, Professor, Department of Statistics and Department of Machine Learning, CMU and author of All of Statistics

Accessibility: It's freely available online in EdX Mooc (R) or PDF download for Python or R

Practicality: The course includes hands-on exercises and real-world examples, enhancing practical understanding.