add https://allcontributors.org/

[fdiblen]

originally suggested by @abelsiqueira here : https://github.com/NLeSC/generalization/issues/265

[abelsiqueira]

This is the project: https://allcontributors.org

Workflow:

• On the Pull Request or issue you add a message like " add for " and the user is added to a table of contributors like the one below.
  

This is particularly useful for projects that do not add all contributors to CITATION.cff, but it is also helpful otherwise because you

• specify what each contributor did;
• gives immediate visibility to the user;
• makes the project more inviting to different kinds of contributions.