update doc -- segementation fault referring to FAQ (#480)

* update install with FAQ

* update install with FAQ

* update install with FAQ

* update install with FAQ

* update install with FAQ

* update install with FAQ

README.md

@@ -31,7 +31,9 @@ The tool dispatches and runs trial jobs generated by tuning algorithms to search
 ```bash	
     python3 -m pip install --user --upgrade nni
 ```
-Note: If you are in docker container (as root), please remove `--user` from the installation command.
+* Note:
+  * If you are in docker container (as root), please remove `--user` from the installation command.
+  * If there is any error like `Segmentation fault`, please refer to [FAQ](doc/FAQ.md)
 
 **Install through source code**
 * We support Linux (Ubuntu 16.04 or higher), MacOS (10.14.1) in our current stage. 

docs/FAQ.md

+This page is for frequent asked questions and answers.
+
+
+### tmp folder fulled
+nnictl will use tmp folder as a temporary folder to copy files under codeDir when executing experimentation creation. 
+When met errors like below, try to clean up **tmp** folder first.
+> OSError: [Errno 28] No space left on device
+
+### Cannot get trials' metrics in OpenPAI mode
+In OpenPAI training mode, we start a rest server which listens on 51189 port in nniManager to receive metrcis reported from trials running in OpenPAI cluster. If you didn't see any metrics from WebUI in OpenPAI mode, check your machine where nniManager runs on to make sure 51189 port is turned on in the firewall rule. 
+
+### Segmentation Fault (core dumped) when installing from source code
+> make: *** [install-XXX] Segmentation fault (core dumped)
+There are two options:
+
+* Update or reinstall you current python's pip like `python3 -m pip install -U pip`
+* Install nni with --no-cache-dir flag like `python3 -m pip install nni --no-cache-dir`
+
+### Job management error: getIPV4Address() failed because os.networkInterfaces().eth0 is undefined.
+Your machine don't have eth0 device, please set nniManagerIp in your config file manually. [refer](https://github.com/Microsoft/nni/blob/master/docs/ExperimentConfig.md)
+
+### Exceed the MaxDuration but didn't stop
+When the duration of experiment reaches the maximum duration, nniManager will not create new trials, but the existing trials will continue unless user manually stop the experiment. 
+
+### Could not stop an experiment using `nnictl stop`
+If you upgrade your nni or you delete some config files of nni when there is an experiment running, this kind of issue may happen because the loss of config file. You could use `ps -ef | grep node` to find the pid of your experiment, and use `kill -9 {pid}` to kill it manually.
+
+### Could not get `default metric` in webUI of virtual machines
+Config the network mode to bridge mode or other mode that could make virtual machine's host accessible from external machine, and make sure the port of virtual machine is not forbidden by firewall. 
+
+### Help us improve
+Please inquiry the problem in https://github.com/Microsoft/nni/issues to see whether there are other people already reported the problem, create a new one if there are no existing issues been created.