[ffigen] Update readme to clarify C support. (#209)

We will not support C++ headers, because C++ does not have a stable ABI like C (or use of extern "C" in C++ headers). Without a stable ABI it is impossible to know in what register/stack locations to pass arguments and this will lead to endless segmentation faults.

We could potentially support C++ headers which have extern "C" on every symbol, but we will not support C++ headers.

pkgs/ffigen/README.md

diff --git a/pkgs/ffigen/README.md b/pkgs/ffigen/README.md
index a008d37..099fe04 100644
--- a/pkgs/ffigen/README.md
+++ b/pkgs/ffigen/README.md
@@ -4,6 +4,8 @@
 
 Binding generator for [FFI](https://dart.dev/guides/libraries/c-interop) bindings.
 
+> Note: ffigen only supports parsing `C` headers.
+
 ## Example
 
 For some header file _example.h_: