Suggestion to add Megatron to PYTHONPATH

Explains how to add fix a ModuleNotFoundError by adding Megatron to the PYTHONPATH

README.md

@@ -118,6 +118,22 @@ wget --content-disposition https://api.ngc.nvidia.com/v2/models/nvidia/megatron_
 python src/transformers/models/megatron_gpt2/convert_megatron_gpt2_checkpoint.py megatron_lm_345m_v0.0.zip
 ```
 
+As explained by Stas Bekman in [PR #14956](https://github.com/huggingface/transformers/pull/14956), if when running this conversion 
+script and you're getting an exception:
+```
+ModuleNotFoundError: No module named 'megatron.model.enums'
+```
+you need to tell python where to find the clone of Megatron-LM, e.g.:
+```
+cd /tmp
+git clone https://github.com/NVIDIA/Megatron-LM
+PYTHONPATH=/tmp/Megatron-LM python src/transformers/models/megatron_bert/convert_megatron_bert_checkpoint.py ...
+```
+Or, if you already have it cloned elsewhere, simply adjust the path to the existing path.
+
+If the training was done using a Megatron-LM fork, e.g. [Megatron-DeepSpeed](https://github.com/microsoft/Megatron-DeepSpeed/) then 
+you may need to have that one in your path, i.e., /path/to/Megatron-DeepSpeed.
+
 3. Fetch missing files
 ```
 git clone https://huggingface.co/nvidia/megatron-gpt2-345m/