Docs(PR): Document the process of opening a PR

[christian-pinto]

Description

Document what is the process for a contributor to open a PR to add a Nexus Package to algorithm nexus:

• How to add python dependencies for the main python package, testing and benchmarking to the Nexus dependencies
• How to prepare and validate a Nexus Package, including folders, config files and test files.
• Document any other additional information the is required for us to process a PR

Motivation

We need a well structured PR process to help developers minimize the iterations to get their algorithm integrated in this project.