Add rules to homepage

[computer-geek64]

The two main rules should be added to the homepage or subpage so users can better understand the competition guidelines.

It should clearly articulate how:

1. Need to complete at least 2 easies or 1 medium problem to be considered "done" for the day - 1 easy is not sufficient.
2. Cannot re-solve a problem that has been previously completed within the last year. Only the first valid solution will be counted for at least 1 year.

[computer-geek64]

Probably also useful to denote the midnight UTC cutoff for problems, which is based on LeetCode's definition of a day