Documentation for the tool-side interface

[wrwilliams]

As near as I can tell, there are two options for tool authors wishing to provide an NVTX implementation (such that NVTX annotations can be recorded in another format, as opposed to converting their own annotations to NVTX):

1. Implement an independent NVTX library against the headers
2. Some sort of export table mechanism, wherein we could provide pointers to the functions we wish to implement and a function that registers those function pointers.

The mechanism in 2. is sadly not documented anywhere I've been able to find online (notably not in the NVTX Doxygen docs) and the comments are somewhat sparse for a third party implementor. Is there any chance this interface could become explicitly documented/have I simply missed something in my initial research?

[evanramos-nvidia]

Related to #104