Add a Troubleshooting.md file

doc/troubleshooting/Troubleshooting.md

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+## Troubleshooting
+1. The most common problem is whether two settings correspond with each other, including:
+    - The order of elements in `type_map` and `mass_map` and **`fp_pp_files`**.
+    - Size of `init_data_sys` and `init_batch_size`.
+    - Size of `sys_configs` and `sys_batch_size`.
+    - Size of `sel_a` and actual types of atoms in your system.
+    - Index of `sys_configs` and `sys_idx`.
+
+2. Please verify the directories of `sys_configs`. If there isn't any POSCAR for `01.model_devi` in one iteration, it may happen that you write the false path of `sys_configs`.
+3. Correct format of JSON file.
+4. The frames of one system should be larger than `batch_size` and `numb_test` in `default_training_param`. It happens that one iteration adds only a few structures and causes error in next iteration's training. In this condition, you may let `fp_task_min` be larger than `numb_test`.
+