update readme and templates to clarify scope (#1437)

Co-authored-by: Juan Font <juanfontalonso@gmail.com>

.github/pull_request_template.md

@@ -1,3 +1,15 @@
+<!--
+Headscale is "Open Source, acknowledged contribution", this means that any
+contribution will have to be discussed with the Maintainers before being submitted.
+
+This model has been chosen to reduce the risk of burnout by limiting the
+maintenance overhead of reviewing and validating third-party code.
+
+Headscale is open to code contributions for bug fixes without discussion.
+
+If you find mistakes in the documentation, please submit a fix to the documentation.
+-->
+
 <!-- Please tick if the following things apply. You… -->
 
 - [ ] read the [CONTRIBUTING guidelines](README.md#contributing)