Add "Why Does This Exist" to readme.md

[bai-yi-bai]

Hi @tjko,
As I get started on launching my own hardware projects on GitHub, I think a lot about the readme.md file, especially about the justification, or "Why does this project exist? Why do I need to include fan control circuitry to my projects? Why can't I just go buy a ~5 USD SATA to fan controller?" Of course, I'm asking questions which you know the answer to, but the general audience I'm describing my not.

While the fanpico is getting a lot of stars and interest, I think it has a lot of potential to be embraced by the PC gaming enthusiast community. So, my suggestion here is to create a "Why Does This Exist?" heading, and insert it before the "What is this for?" heading. For example, those cheap fan controllers have a tendency to stop working after a while due to cheap construction, cheap components, and poorly thought out design (traces too thin). In fact, I think I heard this on some youtube videos. The next step would then be to maybe share it with some YouTubers focusing on PC hardware.