Create PULL_REQUEST_TEMPLATE.md
Proposing to copy this neat feature from pytorch. This is a small template that let's a PR submitter tell which issue that PR closes.

Here is an example of it in action, the end of the top post: https://github.com/huggingface/transformers/pull/6659
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6660?src=pr&el=h1) Report
> Merging [#6660](https://codecov.io/gh/huggingface/transformers/pull/6660?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0f94151dc7809128b40ab68ba164742fe1c5b4e6?el=desc) will **increase** coverage by `0.64%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6660/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6660?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6660      +/-   ##
==========================================
+ Coverage   79.01%   79.65%   +0.64%     
==========================================
  Files         156      156              
  Lines       28248    28248              
==========================================
+ Hits        22320    22502     +182     
+ Misses       5928     5746     -182     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6660?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6660/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.81%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6660/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.64% <0.00%> (+1.30%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6660/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `98.95% <0.00%> (+73.82%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6660?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6660?src=pr&el=footer). Last update [0f94151...73edfdc](https://codecov.io/gh/huggingface/transformers/pull/6660?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
