Chore: improve contribution guide (#4379)

Author: georgesittas

CONTRIBUTING.md

@@ -9,43 +9,61 @@ easy and transparent as possible, whether it's:
 - Proposing new features
 
 ## We develop with Github
+
 We use github to host code, to track issues and feature requests, as well as accept pull requests.
 
+## Finding tasks to work on
+
+When the core SQLGlot team does not plan to work on an issue, it is usually closed as "not planned". This may happen
+when a request is exceptionally difficult to address, or because the team deems that it shouldn't be prioritized.
+
+These issues can be a good starting point when looking for tasks to work on. Simply filter the issue list to fetch
+the closed issues and then search for those marked as "not planned". If the scope of an issue is not clear or you
+need guidance, feel free to ask for clarifications.
+
+Before taking on a task, consider studying the [AST primer](https://github.com/tobymao/sqlglot/blob/main/posts/ast_primer.md) and the [onboarding document](https://github.com/tobymao/sqlglot/blob/main/posts/onboarding.md).
+
 ## Submitting code changes
-Pull requests are the best way to propose changes to the codebase. We actively welcome your pull requests:
 
-Please keep PR's small and do your best to follow the conventions of the project. If you have a feature that requires a lot of code changes,
-please reach out to us on [Slack](https://tobikodata.com/slack) before making a PR. This will increase the chances of your PR getting in.
+Pull requests are the best way to propose changes to the codebase, and we actively welcome them.
 
-1. Fork the repo and create your branch from `main`.
-2. If you've added code that should be tested, add tests.
-3. If you've changed APIs, update the documentation.
-4. Ensure the test suite & linter [checks](https://github.com/tobymao/sqlglot/blob/main/README.md#run-tests-and-lint) pass.
-5. Issue that pull request and wait for it to be reviewed by a maintainer or contributor!
+Pull requests should be small and they need to follow the conventions of the project. For features that require
+many changes, please reach out to us on [Slack](https://tobikodata.com/slack) before making a request, in order
+to share any relevant context and increase its chances of getting merged.
+
+1. Fork the repo and create your branch from `main`
+2. If you've added code with non-trivial changes, add tests
+3. If you've changed APIs, update the documentation (docstrings)
+4. Ensure the test suite & linter [checks](https://github.com/tobymao/sqlglot/blob/main/README.md#run-tests-and-lint) pass
+5. Issue that pull request and wait for it to be reviewed by a maintainer or contributor
 
 Note: make sure to follow the [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) guidelines when creating a PR.
 
 ## Report bugs using Github's [issues](https://github.com/tobymao/sqlglot/issues)
+
 We use GitHub issues to track public bugs. Report a bug by opening a new issue.
 
 **Great Bug Reports** tend to have:
 
 - A quick summary and/or background
 - Steps to reproduce
-  - Be specific!
+  - Be specific
   - Give sample code if you can
 - What you expected would happen
 - What actually happens
 - Notes (possibly including why you think this might be happening, or stuff you tried that didn't work)
 - References (e.g. documentation pages related to the issue)
 
 ## Start a discussion using Github's [discussions](https://github.com/tobymao/sqlglot/discussions)
+
 [We use GitHub discussions](https://github.com/tobymao/sqlglot/discussions/190) to discuss about the current state
-of the code. If you want to propose a new feature, this is the right place to do it! Just start a discussion, and
+of the code. If you want to propose a new feature, this is the right place to do it. Just start a discussion, and
 let us know why you think this feature would be a good addition to SQLGlot (by possibly including some usage examples).
 
 ## [License](https://github.com/tobymao/sqlglot/blob/main/LICENSE)
+
 By contributing, you agree that your contributions will be licensed under its MIT License.
 
 ## References
+
 This document was adapted from [briandk's template](https://gist.github.com/briandk/3d2e8b3ec8daf5a27a62).