Add doc for sanitizer flags - #84

It is a little more involved to get the sanitizers working for node.js projects. This is because we either need to:

• compile the nodejs binary itself with -fsanitize=address,undefined,integer; or
• we need to preload the address sanitizer shared library (on OS X at least) to get it to work without the nodejs binary being instrumented.

So the blurb you've added will work great only in the case that:

• A) We're compiling pure C++
• B) A recent clang++ or g++ is on PATH that supports the sanitizers.

B will not be the case on OS X unless we install clang++ via mason or homebrew (because Apple's default clang++ does not support al the sanitizers).

So, all these things combined mean that I think we should provide a custom make sanitize command to bake all the puzzle pieces into a single command and hide the complexity.