docs: Link to gRPC PyPy GitHub issue

Author: bkeryan

README.rst

@@ -57,7 +57,8 @@ Python Version Support
 
 **nidaqmx** supports CPython 3.9+.
 
-**nidaqmx** supports PyPy3 for non-gRPC use cases.
+**nidaqmx** supports PyPy3 for non-gRPC use cases. For the status of PyPy support for gRPC,
+see `PyPy support (grpc/grpc#4221) <https://github.com/grpc/grpc/issues/4221>`_.
 
 Installation
 ============