Add github issue templates and PR template

.github/pull_request_template.md

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+<!-- You can erase any parts of this template not applicable to your Pull Request. -->
+
+### Description
+
+<!-- Describe the purpose of this PR, what's being adding and/or fixed -->
+
+### Notes to the reviewers
+
+<!-- In this section you can include notes directed to the reviewers, like explaining why some parts
+of the PR were done in a specific way -->
+
+### Checklists
+
+#### All Submissions:
+
+* [ ] I've signed all my commits
+* [ ] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
+* [ ] I ran `cargo fmt` and `cargo clippy` before committing
+
+#### New Features:
+
+* [ ] I've added tests for the new feature
+* [ ] I've added docs for the new feature
+* [ ] I've updated `CHANGELOG.md`
+
+#### Bugfixes:
+
+* [ ] This pull request breaks the existing API
+* [ ] I've added tests to reproduce the issue which are now passing
+* [ ] I'm linking the issue being fixed by this PR