readme: mention bootstrap.sh in installation/compiliation notes

as discussed at
https://github.com/jessek/hashdeep/issues/288#issuecomment-17392268

me:
> this step (running bootstrap.sh) is not necessary under all circumstances.
> when is it necessary?

simsong:
> It's necessary if you clone the development repository
> or download a snapshot with the github download.
> We don't put the files generated with autoconf or automake into the repository
> because there would be too many errors.