Add PR template

.github/pull_request_template.md

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+Applicable spec: <link>
+
+### Overview
+
+<!-- A high level overview of the change -->
+
+### Rationale
+
+<!-- The reason the change is needed -->
+
+### Juju Events Changes
+
+<!-- Any changes to the juju events being observed (newly added, significantly modified or deleted) -->
+
+### Module Changes
+
+<!-- Any high level changes to modules and why (Service, Observer, helper) -->
+
+### Library Changes
+
+<!-- Any changes to charm libraries -->
+
+### Checklist
+
+- [ ] The [charm style guide](https://juju.is/docs/sdk/styleguide) was applied
+- [ ] The [contributing guide](https://github.com/canonical/is-charms-contributing-guide) was applied
+- [ ] The changes are compliant with [ISD054 - Managing Charm Complexity](https://discourse.charmhub.io/t/specification-isd014-managing-charm-complexity/11619)
+
+<!-- Explanation for any unchecked items above -->