Document how to contribute new database entry via YAML issue template

[weiji14]

Contributing a new satellite-sensor database entry just got easier with #33. Now let's document it in CONTRIBUTING.md!

Steps:

1. Go to https://github.com/APECS-Earth-Observation/Polar-EO-Database/issues/new/choose and select 'New Database Entry'
2. Provide instructions on filling up the form:

3. Mention what comes afterwards

• edits can be made to the submitted form/issue
• the feedback/review process from repository maintainers
• etc

TODO after full automation is completed in #35. This PR extends #9.