Two inaugural FAQ questions (facebookresearch#3073)

* Two inaugural FAQ questions

* Third question

docs/source/faq.md

@@ -1,5 +1,18 @@
 # Frequently Asked Questions
 
-This document contains a number of questions that are regularly asked on GitHub
-Issues.
+This document contains a number of questions that are regularly asked on GitHub Issues.
 
+
+**Why is my model not generating a response?**
+
+For a generative model, check that `--skip-generation` is set to `False`.
+
+
+**Why can't I reproduce the results of an evaluation on a task with a pretrained model?**
+
+One common culprit for this is that the flags for that task may not be correctly set. When loading a pretrained checkpoint, all of the parameters for the model itself will be loaded from the model's `.opt` file, but all task-specific parameters will need to be re-specified.
+
+
+**Why is my generative model's perplexity so high (>1000) when evaluating?**
+
+One first thing to check is whether there is a problem with your dictionary or token embeddings, because this high perplexity implies that the model is very bad at predicting the next token in a string of text.