docs: add CONTRIBUTING.md docs (#1834)

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,38 @@
+<!--
+    Please give your PR a title in the form "area: short description".  For example "cpu: reduce usage by 95%" or "docs: fix typo in installation.md".
+
+    If your PR is to fix an issue, put "Fixes #issue-number" in the description.
+
+    Don't forget!
+
+    - Please sign CNCF's Developer Certificate of Origin and sign-off your commits by adding the -s / --signoff flag to `git commit`. See https://github.com/apps/dco for more information.
+
+    - If the PR adds or changes a behaviour or fixes a bug of an exported API it would need a unit/e2e test.
+
+    - Performance improvements would need a benchmark test to prove it.
+
+    - All comments should start with a capital letter and end with a full stop.
+ -->
+
+
+#### What this PR does / why we need it
+
+#### Which issue this PR fixes
+
+*(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
+
+- fixes #
+
+#### Special notes for your reviewer
+
+#### Particularly user-facing changes
+
+#### Checklist
+
+Complete these before marking the PR as `ready to review`:
+
+<!-- [Place an '[x]' (no spaces) in all applicable fields.] -->
+
+- [ ] [DCO](https://github.com/prometheus-community/helm-charts/blob/main/CONTRIBUTING.md#sign-off-your-work) signed
+- [ ] The PR title has a summary of the changes and the area they affect
+- [ ] The PR body has a summary to reflect any significant (and particularly user-facing) changes introduced by this PR