Provide issue template(s) for people to seek feedback on "NWB enhancement" possible new NDX

[yarikoptic]

github allows for templating entry of different types of issues. See e.g. https://github.com/bids-standard/bids-specification/tree/master/.github/ISSUE_TEMPLATE for various types BIDS has.

@rly mentioned that it is typical for people to first ask here on either adding a specific feature (new data type etc) would require an NDX or possibly just done within nwb-schema. So I thought it might be nice to have a dedicated template for such enhancements which would provide desired structure to such an issue and instruct submitter to provide necessary details (either new data type is for wide use or just ad-hoc in lab, etc).

[rly]

Just to add:

• for minor bug fixes, documentation changes, and addition of minor metadata fields, we generally recommend submitting an issue or PR here and we'll make the change here.
• for addition of experiment-specific, lab-specific, or device-specific metadata or data types that are not useful to a broad swath of the community, we recommend creating and using an extension.
• for addition of a new data modality, major breaking changes to existing data types, we generally recommend creating an extension and proposing that the extension be merged into core.

Am I missing any use cases?