Add issue ticket link to DistributionMeta (#5355)

Author: canyon289

pymc/distributions/distribution.py

@@ -75,6 +75,17 @@ class _Unpickling:
 
 
 class DistributionMeta(ABCMeta):
+    """
+    DistributionMeta class
+
+
+    Notes
+    -----
+    DistributionMeta currently performs many functions, and will likely be refactored soon.
+    See issue below for more details
+    https://github.com/pymc-devs/pymc/issues/5308
+    """
+
     def __new__(cls, name, bases, clsdict):
 
         # Forcefully deprecate old v3 `Distribution`s