Add a README.txt file in the nilearn_data folder

[GaelVaroquaux]

When we create the nilearn_data folder, we should add a file that says what it is, and that it can be safely deleted.

[kchawla-pi]

Sure. Once we fix all the fetchers and it is indeed safe to delete the data because it can be reliably redownloaded. ;-)

[Remi-Gau]

Sounds like a good first issue.

Here too this may require some pointers for new contributors as to what part of the code should be edited