doc: Add GitHub pr template

.github/PULL_REQUEST_TEMPLATE.md

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+Pull requests without a rationale and clear improvement may be closed
+immediately.
+
+Please provide clear motivation for your patch and explain how it improves
+Bitcoin Core user experience or Bitcoin Core developer experience
+significantly.
+
+* Any test improvements or new tests that improve coverage are always welcome.
+* All other changes should have accompanying unit tests (see `src/test/`) or
+  functional tests (see `test/`). Contributors should note which tests cover
+  modified code. If no tests exist for a region of modified code, new tests
+  should accompany the change.
+* Bug fixes are most welcome when they come with steps to reproduce or an
+  explanation of the potential issue as well as reasoning for the way the bug
+  was fixed.
+* Features are welcome, but might be rejected due to design or scope issues.
+  If a feature is based on a lot of dependencies, contributors should first
+  consider building the system outside of Bitcoin Core, if possible.
+* Refactoring changes are only accepted if they are required for a feature or
+  bug fix or otherwise improve developer experience significantly. For example,
+  most "code style" refactoring changes require a thorough explanation why they
+  are useful, what downsides they have and why they *significantly* improve
+  developer experience or avoid serious programming bugs. Note that code style
+  is often a subjective matter. Unless they are explicitly mentioned to be
+  preferred in the [developer notes](/doc/developer-notes.md), stylistic code
+  changes are usually rejected.
+
+Bitcoin Core has a thorough review process and even the most trivial change
+needs to pass a lot of eyes and requires non-zero or even substantial time
+effort to review. There is a huge lack of active reviewers on the project, so
+patches often sit for a long time.