Add LICENSE

[TjWallas]

Please add an appropriate LICENSE file for what others are allowed to do with the source code and material listed here. Furthermore, please add due attribution to all contributors, if there are multiple collaborators who helped this work come about.

[AhmadElsagheer]

added license, thanks! Could you explain further adding attribution?

[TjWallas]

@AhmadElsagheer 👍

By that, I mean listing the names of the individuals who:

• Authored/co-authored the problems (e.g. From Codeforces, ACM or other platforms).
• Helped direct/supervise you either verbally (e.g. Fruitful discussions) or via coaching you in the first place.
• Helped write solutions/source code to these problems.
• Helped write tests to validate/invalidate/rank solutions.
• Helped improve the quality of the material in this repository through PRs.
• Old theoretical and/or practical material you consulted to categorize and/or find solutions to these problems.
• ... etc)