Revise project documentation.

Hello, DemocracyOS team!

This PR proposes changes on the current documentation, specifically, `CHANGELOG`, `CONTRIBUTING`, `CONTRIBUTORS` and `README` pages. These are mostly clarifications and typographic corrections, but it also includes:
- a link to the `CONTRIBUTING` page on the `README`
- the MIT license text and reference that were missing
- the replacement of the contributors list from the `README` for a separate, new `CONTRIBUTORS` page
- the renaming of the ChangeLog from `History` to `CHANGELOG`, for the latter being more conventional among open source projects.

What do you think of this?