Add "Train on Valohai" buttons to README
This pull request adds two "Train on Valohai" buttons to README:

![Screenshot from 2019-12-19 16-59-54](https://user-images.githubusercontent.com/2681608/71184210-1284b000-2282-11ea-8bb8-72bf93014672.png)

and...

![Screenshot from 2019-12-19 17-00-07](https://user-images.githubusercontent.com/2681608/71184213-13b5dd00-2282-11ea-84c7-393afbbae7ea.png)

When clicked, it will automatically create a project on Valohai and let you run the project examples without any further setup. Effectively the same as "Deploy to Heroku" button if you are familiar with that.

## The flow looks like this (after login):

![Screenshot from 2019-12-19 17-00-19](https://user-images.githubusercontent.com/2681608/71184512-88891700-2282-11ea-855b-5a187ef2e700.png)
![Screenshot from 2019-12-19 17-00-34](https://user-images.githubusercontent.com/2681608/71184528-8d4dcb00-2282-11ea-8228-6322236360a9.png)
![Screenshot from 2019-12-19 17-05-31](https://user-images.githubusercontent.com/2681608/71184531-8e7ef800-2282-11ea-970a-aefa5d127069.png)
![Screenshot from 2019-12-19 17-06-18](https://user-images.githubusercontent.com/2681608/71184536-9048bb80-2282-11ea-98b0-7fd371a78677.png)

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2227?src=pr&el=h1) Report
> Merging [#2227](https://codecov.io/gh/huggingface/transformers/pull/2227?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/62c1fc3c1ecdfab787ee3c34d1ec1eba65c18877?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2227/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2227?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2227   +/-   ##
=======================================
  Coverage   81.47%   81.47%           
=======================================
  Files         122      122           
  Lines       18344    18344           
=======================================
  Hits        14946    14946           
  Misses       3398     3398
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2227?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/2227?src=pr&el=footer). Last update [62c1fc3...6bfc181](https://codecov.io/gh/huggingface/transformers/pull/2227?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Big fan of the images explaining the usage :)<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
