docs: Addition of the section for custom dataset

[khalidw]

This section titled "For custom dataset" includes a guide on how to calculate MOT metrics for custom dataset.

[cheind]

As requested by #131

[cheind]

@khalidw looks fine to me. Looking for somone that actually applies your recipe for confirmation to merge. One remark: would you mind updating the Readme to better reflect when to use the tool you advertise and when this is unnecessary?

[khalidw]

@cheind yeah it's fine. I used this code for my Master's thesis back in August 2021. It would be great if someone else can confirm.

[cheind]

One remark: would you mind updating the Readme to better reflect when to use the tool you advertise and when this is unnecessary?

[cheind]

thanks!