Add section for Constructive Algorithms

[Awadesh365]

Cf Article- https://codeforces.com/blog/entry/80317

e.g. https://assets.hkoi.org/training2019/cast.pdf https://codeforces.com/problemset/problem/1405/B

[bqi343]

The tips in the articles aren't specific to constructive problems; solving small cases and finding patterns is always a reasonable thing to try ...

[Awadesh365]

got it.