Add section on contribution etiquette

Co-authored-by: Ravin Kumar <[email protected]>
Co-authored-by: Oriol Abril <[email protected]>

Author: 3 people

CONTRIBUTING.md

@@ -17,7 +17,17 @@ Please verify that your issue is not being currently addressed by other issues o
 
 Filter on the ["beginner friendly"](https://github.com/pymc-devs/pymc/issues?q=is%3Aopen+is%3Aissue+label%3A%22beginner+friendly%22) label for issues which are good for new contributors.
 
-# Contributing code via pull requests
+## Etiquiette for code contributions
+* When you start working working on an issue, open a `Draft` pull request as soon as you're doing the first steps (see steps below).
+* Before opening a PR with a new feature, please propose it in an [Issue](https://github.com/pymc-devs/pymc/issues) or [Discussion](https://github.com/pymc-devs/pymc/discussions) with the maintainers. Depending on the proposal we might direct you to other places such as [`pymc-experimental`](https://github.com/pymc-devs/pymc-experimental) or [`pymc-examples`](https://github.com/pymc-devs/pymc-examples).
+* Any issue without an open pull request is available for work.
+   * If a pull request has no recent activity it may be closed, or taken over by someone else.
+   * The specific timeframe for "recent" is hard to define as it depends on the contributor the specific code change, and other contextual factors. As a rule of thumb in a normal pull request with no other blockers there is typically activity every couple of days.
+   * The core devs will make their best judgement when opting to close PRs.
+* In accordance with the previous point, please don't make unrealistic "_I want to work on this issue_" pledges.
+* It's okay if priorities change, but please communicate if you can't get a PR to the finish line, or that you're taking a break. Depending on the change (urgent bugfix vs. new feature) others might want to take over to get the work done.
+
+## Contributing code via pull requests
 
 While issue reporting is valuable, we strongly encourage users who are inclined to do so to submit patches for new or existing issues via pull requests. This is particularly the case for simple fixes, such as typos or tweaks to documentation, which do not require a heavy investment of time and attention.
 
@@ -27,6 +37,8 @@ The preferred workflow for contributing to PyMC is to fork the [GitHub repositor
 
 ### Steps
 
+1. Read the [Contributing etiquette section](#etiquiette-for-code-contributions).
+
 1. Fork the [project repository](https://github.com/pymc-devs/pymc/) by clicking on the 'Fork' button near the top right of the main repository page. This creates a copy of the code under your GitHub user account.
 
 1. Clone your fork of the PyMC repo from your GitHub account to your local disk, and add the base repository as a remote: