Document support for v1 moving forwards (#2124)

Author: glbrntt

README.md

@@ -12,6 +12,25 @@ major version, v2.x. Pre-release versions will be available in the near future.
 In the meantime, v1.x is available and still supported. You can read more about
 it on the [Swift Package Index][spi-grpc-swift-main].
 
+## Support
+
+As gRPC Swift v2.x is being developed, v1.x will continue to be supported.
+However, the support window for v1.x will decrease over time as new Swift
+versions are released.
+
+From the next Swift release, the number of Swift versions supported by
+gRPC Swift v1.x will decrease by one each time.
+
+Assuming the next Swift releases are 6.1, 6.2, 6.3, and 6.4 then the versions of
+Swift supported by gRPC Swift are as follows.
+
+Swift Release | Swift versions supported by 1.x
+--------------|--------------------------------
+6.1           | 5.10, 6.0, 6.1
+6.2           | 6.1, 6.2
+6.3           | 6.3
+6.4           | Unsupported
+
 ## Security
 
 Please see [SECURITY.md](SECURITY.md).