Add a new category Sliding Window and also include the question on longest substring with k unique characters.

[MuditWadhwa]

Hi, I would like to contribute to the project by making a new folder for sliding window technique. I would also like to add a question on the same. Please assign this issue to me and add Hacktoberfest label to it.

[avinash201199]

@MuditWadhwa
wait for the first one , will assing you .

[MuditWadhwa]

I would like to contribute a question in the sliding window category(and add the sliding window folder as well). Please assign this issue to me and add the Hacktoberfest label as well.