added code for distinct ways to climb stair problem

[find3rskeeper]

Added a file named distinct_ways_to_climb_stairs.cpp containing

1. The question at the top of the file.
2. Code solving the problem statement

Related issue #124

Solved the question on interviewbit with all the test cases passing.

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (change which fixes an issue with the algorithm)

• New Algorithm (non-breaking change which adds functionality)

• Documentation

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

If you're unsure about any of these, don't hesitate to ask. We're here to help!

• My code follows the code style of this project.

• My change requires a change to the documentation.

• I have updated the documentation accordingly.

• I have read the CONTRIBUTING document.

[tstreamDOTh]

Add proper directory structure. Check other examples

[find3rskeeper]

@tstreamDOTh added proper directory structure

[tstreamDOTh]

Lgtm