Add version macro

Author: BenBrock

markdown/intro.md

@@ -1,2 +1,15 @@
 # Intro
 Hello, this is the intro.
+
+### Version Conformance
+A GraphBLAS library should define the macro `GRAPHBLAS_CPP_VERSION` to indicate its
+level of conformance with the GraphBLAS C++ API.  To indicate conformance with the
+GraphBLAS 0.1 draft specification, a library should provide the macro `GRAPHBLAS_CPP_VERSION`
+with a value greater than or equal to `202207L`.
+
+#### Exposition Only
+```cpp
+// Define the `GRAPHBLAS_CPP_VERSION` macro to indicate
+// conformance to the GraphBLAS 0.1 Draft Specification.
+#define GRAPHBLAS_CPP_VERSION 202207L
+```