Adding pull request template

.github/pull_request_template.md

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+## Describe your changes
+
+## Checklist before requesting a review
+
+- [ ] I have performed a self-review of my code.
+- [ ] I have added thorough tests with at **least** 90% code coverage.
+- [ ] I've modified or created example GOBL documents the show my changes in use, if appropriate.
+- [ ] When adding or modifying a tax regime or addon, I've added links to the source of the changes either structured or in the comments.
+- [ ] I've run `go generate .` to ensure that Schemas and Regime data are up to date.
+- [ ] All linter warnings have been reviewed and fixed.
+- [ ] I've been obsessive with pointer nil checks to avoid panics.
+- [ ] Requested a review from @samlown.
+