Name wrangling c++ caveat in Readme (#511)

* Name wrangling c++ caveat in Readme

Try to add a small help for people not super familiar in C++ and name wrangling.

* Update README.md

* Update README.md

Author: Narsil

README.md

@@ -150,6 +150,7 @@ fn main() {
 When using C++ library compilation switch, the `CXX` and `CXXFLAGS` env
 variables are used instead of `CC` and `CFLAGS` and the C++ standard library is
 linked to the crate target.
+Remember that C++ does name mangling so `extern "C"` might be required to enable rust linker to find your functions.
 
 ## CUDA C++ support