Better documentation on f3write

[kwinz]

I was looking for what data f3write writes to the drive. The README.md only states that

f3write will write large files to your mounted disk

But are those predictable? Random? Just 0xFF? There are actually tools like stressdisk that write the same 1GB file until the disk is full. So the documentation was no help and I had to read the source code.

In https://github.com/AltraMayor/f3/blob/master/f3write.c#L131 I found sector[i] = random_number(sector[i - 1]);. And finally in tools.h I found static inline uint64_t random_number(uint64_t prv_number) {return prv_number * 4294967311ULL + 17;}

So please add to the documentation that

f3write will write large files of pseudorandom data to your mounted disk

Thank you!