pkg-Read-me

Stan-specific notes:

* All '.stan' files containing stanmodel definitions must be placed in 'inst/stan'.
* Additional files to be included by stanmodel definition files
  (via e.g., #include "mylib.stan") must be placed in any subfolder of 'inst/stan'.
* Additional C++ files needed by any '.stan' file must be placed in 'inst/include',
  and can only interact with the Stan C++ library via '#include' directives
  placed in the file 'inst/include/stan_meta_header.hpp'.
* The precompiled stanmodel objects will appear in a named list called 'stanmodels',
  and you can call them with e.g., 'rstan::sampling(stanmodels$foo, ...)'