Add Random Forest FAQ crash solution (#692)

docs/FAQ.md

@@ -74,6 +74,12 @@ Remember this is a free/open community support. We may not be available 24/7 to
 
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+- **Question 7**: I tried to use Random Forest mode, and LightGBM crashes!
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+- **Solution 7**: It is by design. You must use `bagging_fraction` and `feature_fraction` different from 1, along with a `bagging_freq`. See [this thread](https://github.com/Microsoft/LightGBM/issues/691) as an example.
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 ### R-package
 
 - **Question 1**: I used `setinfo`, tried to print my `lgb.Dataset`, and now the R console froze!