README-mpers

To use mpers functionality, one should:
* typedef all of the target types which are compound and not typedefed
already;
* for each target type, include DEF_MPERS_TYPE(target_type_t), these can
be included conditionally;
* include MPERS_DEFS once;
* before inclusion of MPERS_DEFS include all important headers
(containing definitions of these types or other behaviour-affecting
defines);
* printers should be defined
as MPERS_PRINTER_DECL(return_type, function_name, args),
inside files that include MPERS_DEFS these printers should be called
as MPERS_FUNC_NAME(function_name)(args), in other files
they should be called just as function_name(args).