Update Polars README.md

Link to tracking issue for source of truth

Author: akshayka

polars/README.md

@@ -4,26 +4,9 @@ _🚧 This collection is a work in progress. Please help us add notebooks!_
 
 This collection of marimo notebooks is designed to teach you the basics of data wrangling using a Python library called Polars.
 
-**Planned notebooks.**
-
-Any notebook with the status "🚧" needs a contributor! Please file an issue if you plan to contribute.
-
-| Notebook               | Description                                                                  | Status | 
-| ---------------------- | ---------------------------------------------------------------------------- | ------ |
-| Why Polars             | An introduction to Polars and its advantages over other Dataframe libraries. | ✅     |
-| Series                 | Homogenous columns of data                                                   | 🚧     |
-| DataFrames             | The central datastructure for structured data                                | 🚧     |
-| Loading data           | Loading data from various different sources                                  | 🚧     |
-| Basic operations       | Basic operations on data                                                     | 🚧     |
-| Working with columns   | Expression expansion and columns                                             | 🚧     |
-| Data types             | Basic types and casting                                                      | 🚧     |
-| Strings                | Working with strings                                                         | 🚧     |
-| Missing data           | Handling missing data                                                        | 🚧     |
-| Aggregations           | Group bys and aggregations                                                   | 🚧     |
-| Window functions       | Advanced expressions with windowing functions                                | 🚧     |
-| User-defined functions | Using custom Python operations                                               | 🚧     |
-| NumPy functions        | Applying NumPy functions to DataFrames                                       | 🚧     |
-| Lazy execution         | Lazily process queries with LazyFrames                                       | 🚧     |
+**Help us build this course! ⚒️**
+
+We're seeking contributors to help us build these notebooks. Every contributor will be acknowledged as an author in this README and in their contributed notebooks. Head over to the [tracking issue](https://github.com/marimo-team/learn/issues/40) to sign up for a planned notebook or propose your own.
 
 **Running notebooks.** To run a notebook locally, use