Write user docs / examples

[Gankra]

Some folks have noted interest in using this but I have no idea what to tell them to even do.

[msirringhaus]

For Linux at least it's basically these functions: https://github.com/rust-minidump/minidump-writer/blob/main/src/linux/minidump_writer.rs#L135-L212

So

let res = MinidumpWriter::new(pid, tid)
                           // ... some options, if you want, like:
                           .set_minidump_size_limit(my_limit)
                           // ... maybe some more options ...
                           .dump(destination)?;

This will write the resulting dump to destination but also returns a Vec<u8>-copy of it in memory to res.