C API: Add SK_API, also documentation of an example.

include/c/sk_types.h

Index: include/c/sk_types.h
diff --git a/include/c/sk_types.h b/include/c/sk_types.h
index 87cc0555bd5697d524f772ebe82d2734012b3d94..f6d73df94d6882a60b760108d3f174a2fe8c98f6 100644
--- a/include/c/sk_types.h
+++ b/include/c/sk_types.h
@@ -23,6 +23,10 @@
     #define SK_C_PLUS_PLUS_END_GUARD
 #endif
 
+#ifndef SK_API
+#define SK_API
+#endif
+
 ///////////////////////////////////////////////////////////////////////////////////////
 
 SK_C_PLUS_PLUS_BEGIN_GUARD
@@ -61,7 +65,7 @@ typedef enum {
     BGR_V_SK_PIXELGEOMETRY,
 } sk_pixelgeometry_t;
 
-sk_colortype_t sk_colortype_get_default_8888();
+SK_API sk_colortype_t sk_colortype_get_default_8888();
 
 typedef struct {
     int32_t         width;