Create pull_request_template.md

Add a template for code review process. It will be included in the upcoming pull requests on this branch.

.github/workflows/pull_request_template.md

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+This is a code review template, it will be displayed after each pull request. 
+We do code reviews because we care about quality of our codebase and because we care about long-term development goals.
+We do not do code reviews to blame the ones who contribute. There is no seniority in this process and there is no perfect code, 
+we just want to avoid storing code that is not OK.
+
+## For the submitter
+
+- [ ] `R CMD build` & `R CMD check` pass
+- [ ]  Needed tests exist and passing or not needed
+- [ ]  The change is sufficiently small
+
+## For the reviewer
+
+- [ ] OK, code does what it's intended for
+- [ ] its' OK to do it that way
+- [ ] it's OK to read and comprehend the code
+- [ ] tests, documentation, style are OK