explain how to successfully run examples in readme and doc
Author: rlouf
Pull request: yes


Comments:
# [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: |

------

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> 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