Update contributing guidelines

docs/sphinx/source/contributing/introduction_to_contributing.rst

@@ -12,6 +12,31 @@ contributors from novice to expert. Don't worry if you don't (yet)
 understand parts of it.
 
 
+.. _core-principles:
+
+Core principles
+~~~~~~~~~~~~~~~
+Some core guidelines on the package, requirements for contributors, and the
+management of contributions are as follows:
+
+* pvlib-python is an advanced scientific modeling toolbox.
+* Every pull request (PR) should address one or more open issues. The PR should
+  be outlined in the issue discussion.
+* GSoC: For Google Summer of Code, we invite applications only from those with
+  significant PV modeling experience and who are motivated to join the pvlib
+  maintenance teams
+* A background in solar energy and/or experience in pvlib-python is required to
+  address some issues effectively due to subtleties that are hard to appreciate
+  until struggling with an implementation. It is not always clear which issues
+  fall into this bucket. Don't be offended if maintainers close your PR for
+  this reason.
+* Maintainers reserve the right to close PRs that do not comply with the
+  contributing guidelines.
+
+While we welcome new contributors, reviewer capacity is limited. Therefore,
+following the guidelines above throughout the contributing process is critical. 
+
+
 .. _easy-ways-to-contribute:
 
 Easy ways to contribute