Add note about supporting GraphBLAS C API version (python-graphblas#441)

* Add notes about supporting GraphBLAS C API version

README.md

@@ -42,6 +42,7 @@ or pip:
 $ pip install python-graphblas[default]
 ```
 This will also install the [SuiteSparse:GraphBLAS](https://github.com/DrTimothyAldenDavis/GraphBLAS) compiled C library.
+We currently support the [GraphBLAS C API 2.0 specification](https://graphblas.org/docs/GraphBLAS_API_C_v2.0.0.pdf).
 
 ### Optional Dependencies
 
@@ -56,7 +57,7 @@ The following are not required by python-graphblas, but may be needed for certai
 ## Description
 Currently works with [SuiteSparse:GraphBLAS](https://github.com/DrTimothyAldenDavis/GraphBLAS), but the goal is to make it work with all implementations of the GraphBLAS spec.
 
-The approach taken with this library is to follow the C-API specification as closely as possible while making improvements
+The approach taken with this library is to follow the C-API 2.0 specification as closely as possible while making improvements
 allowed with the Python syntax. Because the spec always passes in the output object to be written to, we follow the same,
 which is very different from the way Python normally operates. In fact, many who are familiar with other Python data
 libraries (numpy, pandas, etc) will find it strange to not create new objects for every call.

docs/getting_started/faq.rst

@@ -112,6 +112,18 @@ This is motivated by these guidelines:
 For example, if a CVE is discovered, we won't retroactively apply the fix to previous releases.
 Instead, the fix will only be available starting with the next release.
 
+The `GraphBLAS C API specification <https://graphblas.org>`_ is expected to change slowly, but it does change.
+We aim to support the latest version of the GraphBLAS spec and of implementations.
+We will announce plans to drop support of *old* versions of the spec or major versions of implementations
+*before* we do so. We will make the announcements in the
+`release notes <https://github.com/python-graphblas/python-graphblas/releases>`_ and in our Discord channel.
+If the proposed changes will negatively affect you, please
+`let us know <https://github.com/python-graphblas/python-graphblas/issues>`_
+so we may work together towards a solution.
+
+To see which versions of SuiteSparse:GraphBLAS we support, look at the version specification
+of ``suitesparse`` under ``[projects.optional-dependencies]`` in ``pyproject.toml``.
+
 What is the relationship between python-graphblas and pygraphblas?
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++