adding the process to add or remove algorithms to contributing.md

[toscalix]

I have added a whole new section to CONTRIBUTING.md including the following processes:

• Propose the inclusion of a new algorithm
• Propose the deprecation of an existing algorithm
• Propose the removal of an existing algorithm

These processes reference the coming document including the criteria for adding, deprecating or removing algorithms that will determine the decision about the proposals

[toscalix]

After discussing the proposal in the Cryptography Group Meeting, we all agreed in removing the concept of algorithm deprecation from our documentation, at least for the time being.

• The corresponding section has been removed.
• The rest of the new sections has been reviewed to remove any reference to the concept.
• An extra requisite has been added (bullet point) to prevent the use case of standardised algorithm being deprecated by a standardization body, leading to a submission to our list of the nwer standardised algorithm, without any reference to the deprecated algorithm.