Add template for new issues & pull requests

[PallasDoesCode]

Recently, GitHub announced a new feature that allows project maintainers to add information about the project as placeholder text on issues and pull requests.

With the project recently getting attention and with my most recent PR adding a CONTRIBUTING.md aimed at helping newcomers I thought it would be a good idea if we create a template for issues and pull requests that include some basic information on what needs to be included when submitting an issue and what needs to be done prior to submitting the PR.

@yatskevich If you'll include here what information should be added to each I'll be happy to create the templates based on your feedback.

[yatskevich]

For PRs we need to describe our policy of 'clean history' and probably describe how to squash commits before making a PR.

As for issue's template - we can ask for minimal examples of configuration (task config, exportOverrides section, description of a use-case for feature requests).