Documentation for contributor

[lionelkusch]

The documentation is missing one section about Development.

This section should contain:

• the policy for the contribution to the code and the description of our way of working (see https://nilearn.github.io/stable/development.html)
• documentation for maintainers, in order to help to transmit the responsibility to a new person
• the changelog, to trace and inform the user about the modification in each release
• Information about the actual team supporting the maintenance and development of the project and the people responsible for it. For example: the address mail for contact, the way to exchange with the developers, ...
• Some additional information about the technical decision made to help newcomers go inside the code.

[lionelkusch]

A documentation page should contains the decision on the project management.

[lionelkusch]

One part of it will be added with the PR for the new release #258. However, there is a need to be completed later.