Add contributing guidelines for specification issues and MRs

The project's contributing guide should:

• link to and describe the project's communication channels
• describe the legal considerations for contributors, including the ECA and signing commits
• link to the project scope and release plan
• specify what policies apply to the project, including the Code of Conduct, Eclipse IP policy, and EFSP
• describe the information that should be provided in specification issues
• describe/link to how the project makes decisions
• lay out specification merge request guidelines and processes
• provide instructions for setting up a local repository
• list the workflow steps for creating an issue branch and submitting a MR

This issue only focuses on contributions for the specification, not the end user documentation. A separate issue and MR should focus on the end user documentation contributing process.