Added math functions to readme

Added a full list of math functions to README.md. I am not sure
exactly what some of the functions do, so their descriptions is
just '???' at the moment.

Hopefully someone who knows math (statistics?) will come along and
fix this ...
Added memory functions to readme

Added a full list of memory functions to README.md. We are still
missing realloc(), but malloc() and free() should be enough for now.