Add extended information to assist new contributors

[eddieajau]

The contributing document in the Platform repo https://github.com/joomla/joomla-platform/blob/staging/CONTRIBUTING.markdown can be extended to include other sections to assist new or returning contributors (in other words, this document needs to be a roadmap about how to contribute, but not what to contribute). This information may include, but not be limited to:

• Links to the mailing list
• Explanation of when to use the Platform mailing list and the CMS mailing lists
• Notes on when and how to raise a pull request
• Notes on when and how to raise an issue
• Notes on where to find ideas about what you can do
• Links to other pertinent information such as developer.joomla.org, the Platform Manual, ideas.joomla.org (?) and any other relevant policy or general information that would be useful for a contributor. See links on http://developer.joomla.org/code.html for ideas
• Link to the milestones https://github.com/joomla/joomla-platform/issues/milestones
• Ensure that the open/close/re-open pull request policy and instructions are included
• Notes about watching a repository to get notifications
• Notes about where to find help with git

In the process we want to deprecate this document:

https://github.com/joomla/joomla-platform/wiki/Contributing-to-the-Joomla-Platform

If you want to help solve this issue, please edit the above-mentioned file and make a pull request.