add maintainers to numpy dtypes

Author: normanrz

data-types/numpy.datetime64/README.md

@@ -118,3 +118,7 @@ as equivalent representations of the same scalar value (`NaT`).
 ## Codec compatibility
 
 This data type is compatible with any codec that supports arrays of signed 64-bit integers.
+
+## Current maintainers
+
+* [zarr-python core development team](https://github.com/orgs/zarr-developers/teams/python-core-devs)

data-types/numpy.timedelta64/README.md

@@ -121,3 +121,7 @@ as equivalent representations of the same scalar value (`NaT`).
 ## Codec compatibility
 
 This data type is compatible with any codec that supports arrays of signed 64-bit integers.
+
+## Current maintainers
+
+* [zarr-python core development team](https://github.com/orgs/zarr-developers/teams/python-core-devs)