update issue and pr template

Author: juliankrispel

.github/ISSUE_TEMPLATE.md

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+## Use-case/Problem
+
+- If you think a feature should be added to draft-js-plugins please provide a description of the use-case.
+- If you found a bug please reproduce it within a [codesandbox](codesandbox.io), this'll make it easier for us to help you. Here's an [example that uses the mentions plugin](https://codesandbox.io/s/MjG72jX2R) - this will help get you started
+
+Please note: The draft js plugins core team is very small and there's not that much time on our hands. If you can help fix this issue, please do, there's usually someone available in the draft-js-plugins channel [on slack](https://draftjs.herokuapp.com/) for pairing.

.github/PULL_REQUEST_TEMPLATE

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+Please check out Contributing Guidelines. By following this template you help us to review your code.
+https://github.com/draft-js-plugins/draft-js-plugins/blob/master/CONTRIBUTING.md
+
+## Checklist
+
+- [ ] Fix any eslint errors that occur
+- [ ] Update change-log for every plugin you touch
+- [ ] Add/Update tests if you add/change new functionality
+- [ ] Add/Update docs if you add/change functionality
+- [ ] Enable "Allow edits from maintainers" for this PR
+
+## Use-case/Problem
+
+Whether you're fixing a bug or writing a new feature, please open an issue first and discuss with us. We're also reachable on [the draft js slack](https://draftjs.herokuapp.com/)
+
+Briefly describe the use-case/problem you're solving, reference the issue for this bug/feature here
+
+## Implementation
+
+Briefly describe your solution
+
+## Demo
+
+If you're implementing or changing a feature, please add a gif to demo the change, thanks!