Uniformize/update how_to_contribute links in the README.

[Peiffap]

Currently, there are two problems with this:

• The links are not all the same (they refer to the Google Docs link and the DropBox link interchangeably).
• The links are not up to date with the repo; however, simply changing them to the Sharepoint link is not possible, since that would require login access.

At the very least, we should make sure to use the same link everywhere, and ideally, we'd need to find a way to keep those versions of the pdf up to date (worst case scenario: copy them manually every time a commit affects the README, which happens once in a blue moon anyway).

[Peiffap]

Seems like uniformization was easy to do, though the problem of not having an updated document is still open.

[MartinBraquet]

About the update, one could provide an open access link (without email address) to the sharepoint, but it will need to be changed when the onedrive will change owner (that is, soon).

[Peiffap]

[...] but it will need to be changed when the onedrive will change owner [...]

Well, the pdf link would have to be updated already, I think one extra line change isn't going to make too big of a difference.

[...] (that is, soon).

All good things must come to an end... 😭