Dijkstra's algorithm in C++ done

[cs-mshah]

Description

Dijkstra's algorithm in C++ has been done with proper documentation. Kindly verify the pull request.

Reference Issue number #321

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Documentation Update

Checklist:

• [x] I have named my files and folder, according to this project's guidelines.
• [x] My code follows the style guidelines of this project.
• [x] My Pull Request has a descriptive title. (not a vague title like Update index.md)
• [x] I have commented on my code, particularly in hard-to-understand areas.
• [x] I have created a helpful and easy to understand README.md, with problem description and my name.
• [x] My changes do not produce any warnings.
• [x] I have checked for trailing spaces in file names and none have them.