add docs

docs/tutorials/gluon/customop.md

@@ -30,6 +30,7 @@ Custom operator in python is easy to develop and good for prototyping, but may h
 import numpy as np
 import mxnet as mx
 from mxnet import gluon, autograd
+import os
 ```
 
 ## Parameter-less operators
@@ -214,5 +215,20 @@ y = dense(x)
 print(y)
 ```
 
+## Using custom operators with fork
+In Linux systems, the default method in multiprocessing to create process is by using fork. If there are unfinished async custom operations when forking, the program will be blocked because of python GIL.
+
+```
+x = mx.nd.array([0, 1, 2, 3])
+y = mx.nd.Custom(x, op_type='sigmoid')
+os.fork()
+```
+
+Correctly handling this will make mxnet depend upon libpython, so the workaround now is to ensure all custom operation is executed before forking process.
+
+```
+print(y.asnumpy())
+os.fork()
+```
 
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