Update README.md

[aweell]

Updates the README.md file to enhance the documentation regarding contributions to the Mística icons project.

The primary motivation for this change is to encourage community involvement and streamline the process for users who wish to contribute new icons. By adding a dedicated section on how to contribute, along with a link to the contribution guide, we aim to make it easier for users to understand how they can participate in the project.

This improvement not only fosters a collaborative environment but also helps ensure that the icon library can grow and adapt to the needs of its users. Overall, these changes will enhance user engagement and support the ongoing development of Mística icons.

[yceballost]

I would change the link in the first item of the list in this file to ICON_TABLE.md

https://github.com/Telefonica/mistica-icons/blob/production/CONTRIBUTING.md

[yceballost]

This change will be erased in the next readme generation xD

You have to include this in:

mistica-icons/.github/md-generator/generate_markdown.py

Line 268 in d79a8eb

documentation = "" + BREAK + "" + BREAK + BREAK + "Mística Icons is a multibrand icon system that contains all icons that is working in [Mistica Design System](https://github.com/Telefonica/mistica) now. " + BREAK + BREAK + "Mistica support [Brand Factory icons](https://brandfactory.telefonica.com/document/1086#/nuestra-identidad/iconos). This set of icons are a big list of different icons and style that Brand Team worked to be used through Telefonica applications." + BREAK + BREAK + "If you have any question, please you can ask directly in the app of Microsoft Teams, in [Mistica Team](https://teams.microsoft.com/l/team/19%3ad2e3607a32ec411b8bf492f43cd0fe0c%40thread.tacv2/conversations?groupId=e265fe99-929f-45d1-8154-699649674a40&tenantId=9744600e-3e04-492e-baa1-25ec245c6f10). " + \