.github: add PR template

Signed-off-by: paulfantom <pawel@krupa.net.pl>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,37 @@
+<!--
+WARNING: Not using this template will result in a longer review process and your change won't be visible in CHANGELOG.
+-->
+
+## Description
+
+_Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request.
+If it fixes a bug or resolves a feature request, be sure to link to that issue._
+
+
+
+## Type of change
+
+_What type of changes does your code introduce to the kube-prometheus? Put an `x` in the box that apply._
+
+- [ ] `CHANGE` (fix or feature that would cause existing functionality to not work as expected)
+- [ ] `FEATURE` (non-breaking change which adds functionality)
+- [ ] `BUGFIX` (non-breaking change which fixes an issue)
+- [ ] `ENHANCEMENT` (non-breaking change which improves existing functionality)
+- [ ] `NONE` (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)
+
+## Changelog entry
+
+_Please put a one-line changelog entry below. Later this will be copied to the changelog file._
+
+<!-- 
+Your release note should be written in clear and straightforward sentences. Most often, users aren't familiar with
+the technical details of your PR, so consider what they need to know when you write your release note.
+
+Some brief examples of release notes:
+- Add metadataConfig field to the Prometheus CRD for configuring how remote-write sends metadata information.
+- Generate correct scraping configuration for Probes with empty or unset module parameter.
+-->
+
+```release-note
+
+```