Add a CONTRIBUTING guide and an ISSUE_TEMPLATE

[Bisaloo]

• Link to global contributing guide: https://github.com/cran-task-views/ctv/blob/main/Contributing.md
• Mention update cycle & cadence. Errors & archivals are dealt with immediately. New packages are generally added in batches every few months. Issues and PR are tagged with "New package" to confirm they have been seen and will be taken into account.

[Bisaloo]

This should be coupled with an ISSUE_TEMPLATE for request for addition.