[m-dev.] diff: changes to README files

[Simon Taylor]

> Index: .README.in
> ===================================================================
  
> +Some of the files in the `extras' subdirectory (which is now distributed
> +seperately in the `mercury-extras' package) have fairly restrictive
>  copyright licences:  

s/seperately/separately/

> +But in some cases, additional steps may be needed for particular
> +operating systems or particular achitectures, so first read the

s/achitecture/architecture/

Simon.