Better documentation on f3write

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!