add Validate Binary Search Tree

[alexpantyukhin]

Description of Change

Add a guide for new LeetCode solutions.

References

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:

[Panquesito7]

Thanks! 🚀