LayoutLMv2 functionality.
Support for new LayoutLMv2 as discussed in this issue: https://github.com/huggingface/transformers/issues/14160.

Namely, the addition of `LayoutLMv2ForMaskedLM`, which adds a language modeling head on top of the base model to replicate the first pre-training objective in the original paper. 
Hi,

This is actually not the implementation, I see you just added text to a README file. If you want to add `LayoutLMv2ForMaskedLM`, you should implement it in `modeling_layoutlmv2.py` which can be found [here](https://github.com/huggingface/transformers/blob/master/src/transformers/models/layoutlmv2/modeling_layoutlmv2.py).