[readme] add section on use of IVAS libraries and thread-safety

"ISAR_post_rend.exe". All reside in the c-code main directory. 

                       INTEGRATION AS LIBRARIES

                       ========================

While this package contains the necessary applications to execute the IVAS encoder,

decoder, renderer and ISAR post renderer, it is envisioned that the libraries used

would be integrated into custom applications.

It should be noted that this library is not thread-safe by default. Thus, wen using

the IVAS libraries in a mult-threaded environment, proper synchronization of API

calls is required to prevent race conditions by concurrent access to IVAS internal

state memory, FIFO queues buffers or any other data structures. Potential mechanisms

include e.g. mutexes, spinlocks and semaphores. The API calls are at the present not

optimized for fine-granular locking of just critical sections. Some sensitive

sections have thus been marked with a comment in the form /* LOCK XYZ BEGIN */ and

/* LOCK XYZ END */ to provide guidance where code could be modified to prevent

some potential race conditions.

                       RUNNING THE SOFTWARE

                       ====================