Document the dispatcher working dir

docs/en_US/Assessor/CustomizeAssessor.md

@@ -54,6 +54,8 @@ assessor:
 
 Please noted in **2**. The object `trial_history` are exact the object that Trial send to Assessor by using SDK `report_intermediate_result` function.
 
+The working directory of your assessor is `<home>/nni/experiments/<experiment_id>/log`, which can be retrieved with environment variable `NNI_LOG_DIRECTORY`,
+
 More detail example you could see:
 > * [medianstop-assessor](https://github.com/Microsoft/nni/tree/master/src/sdk/pynni/nni/medianstop_assessor)
 > * [curvefitting-assessor](https://github.com/Microsoft/nni/tree/master/src/sdk/pynni/nni/curvefitting_assessor)

docs/en_US/Tuner/CustomizeAdvisor.md

@@ -33,6 +33,8 @@ advisor:
     arg1: value1
 ```
 
+**Note that** The working directory of your advisor is `<home>/nni/experiments/<experiment_id>/log`, which can be retrieved with environment variable `NNI_LOG_DIRECTORY`.
+
 ## Example
 
 Here we provide an [example](https://github.com/microsoft/nni/tree/master/examples/tuners/mnist_keras_customized_advisor).

docs/en_US/Tuner/CustomizeTuner.md

@@ -77,7 +77,7 @@ parameters = {"dropout": 0.3, "learning_rate": 0.4}
 value = 0.93
 ```
 
-**Note that** if you want to access a file (e.g., `data.txt`) in the directory of your own tuner, you cannot use `open('data.txt', 'r')`. Instead, you should use the following:
+**Note that** The working directory of your tuner is `<home>/nni/experiments/<experiment_id>/log`, which can be retrieved with environment variable `NNI_LOG_DIRECTORY`, therefore, if you want to access a file (e.g., `data.txt`) in the directory of your own tuner, you cannot use `open('data.txt', 'r')`. Instead, you should use the following:
 
 ```python
 _pwd = os.path.dirname(__file__)