add PR template (#362)

Author: st--

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,23 @@
+<!-- Comment lines like this one will remain invisible -->
+
+<!-- Thank you for your contribution! Please fill in this template so that we
+can understand your intent and the proposed changes. If anything about this
+template is unclear, just mention it! -->
+
+**Summary**
+<!-- Summary of what & why - explain your motivation and/or link to any GitHub issues this relates to -->
+
+**Proposed changes**
+<!-- Large PRs should ideally be preceded by a design discussion on a separate issue! -->
+
+<!-- A clear and concise description of the contents of this pull request. -->
+* ...
+
+**What alternatives have you considered?**
+<!-- A clear and concise description of any alternative solutions or features you've considered. -->
+
+**Breaking changes**
+<!-- If this PR breaks backwards-compatibility, please start the PR title with `**BREAKING**`! -->
+<!-- In this section, describe any changes that are not backwards-compatible, -->
+<!-- why it is worth breaking backwards compatiblity, -->
+<!-- and how a user would have to address these changes in their downstream code. -->