explain how to successfully run examples in readme and doc
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1889?src=pr&el=h1) Report
> Merging [#1889](https://codecov.io/gh/huggingface/transformers/pull/1889?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/454455c695ff38df1ed3670a43677fdd1abcedf3?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1889/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1889?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1889      +/-   ##
==========================================
+ Coverage   84.05%   84.08%   +0.02%     
==========================================
  Files          97       97              
  Lines       14316    14316              
==========================================
+ Hits        12034    12037       +3     
+ Misses       2282     2279       -3
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1889?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1889/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `97.08% <0%> (+1.45%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1889?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1889?src=pr&el=footer). Last update [454455c...5cd8487](https://codecov.io/gh/huggingface/transformers/pull/1889?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Cool, this is important. I feel like installing from source without cloning (e.g. using `pip install git+https://github.com/huggingface/transformers`) would put less strain on the users' set-up, is there a reason we prefer cloning (I know the aforementioned method also clones, but it puts it in a temporary folder)? This is fine as it is, I'm just curious.<|||||>That way you also cover the case of people who clone the repository to execute the examples (I would believe the majority of users), and encourage this behavior which is a much better practice than copy/pasting.
Doing so you are absolutely sure the version of the library is the exact same as the version of the examples. This may not be true for `pip install git+https://github.com/huggingface/transformers` if the library has since changed.<|||||>> That way you also cover the case of people who clone the repository to execute the examples (I would believe the majority of users), and encourage this behavior which is a much better practice than copy/pasting.
> Doing so you are absolutely sure the version of the library is the exact same as the version of the examples. This may not be true for `pip install git+https://github.com/huggingface/transformers` if the library has since changed.

in fact, passing a branch name, a commit hash, a tag name or a git ref is possible like so:

```
[-e] git://git.example.com/MyProject.git@master#egg=MyProject
[-e] git://git.example.com/MyProject.git@v1.0#egg=MyProject
[-e] git://git.example.com/MyProject.git@da39a3ee5e6b4b0d3255bfef95601890afd80709#egg=MyProject
[-e] git://git.example.com/MyProject.git@refs/pull/123/head#egg=MyProject
```
According to https://pip.pypa.io/en/stable/reference/pip_install/#git <|||||>@LysandreJik Maybe we can list both options i.e. also reference `pip install git+https://github.com/huggingface/transformers`

Also small nitpick, don't hesitate to squash commits when merging very related changes.<|||||>Fair nitpick