update sqlmesh readme (#2782)

* Update README.md

* docs: add some testing steps

warehouse/metrics_mesh/README.md

@@ -2,6 +2,20 @@
 
 ## Setup
 
+You will need [DuckDB](https://duckdb.org/) on your machine. 
+
+Install using Homebrew (macOS/Linux):
+
+```bash
+brew install duckdb
+```
+
+Install using APT (Debian/Ubuntu):
+
+```bash
+sudo apt-get install duckdb
+```
+
 Make sure to set the following environment variables
 in your .env file (at the root of the oso repo)
 
@@ -16,21 +30,92 @@ Make sure you've logged into Google Cloud on your terminal
 gcloud auth application-default login
 ```
 
-Now install dependencies and download playground data into
-a local DuckDB instance.
+Now install dependencies.
 
 ```bash
 poetry install
 poetry shell
+```
+
+Finally, download playground data into your local DuckDB instance with the following command
+
+```bash
 oso metrics local initialize
 ```
 
 ## Run
 
+Run sqlmesh for a sample date range:
+
 ```bash
 cd warehouse/metrics_mesh
 sqlmesh plan dev --start 2024-07-01 --end 2024-08-01 # to run for specific date rates (fast)
-sqlmesh plan  # to run the entire pipeline (slow)
+# sqlmesh plan # to run the entire pipeline (slow)
+```
+
+Explore the data in DuckDB:
+
+```bash
+duckdb
+```
+
+or
+
+```bash
+duckdb /tmp/oso.duckdb
+```
+
+See the tables are loaded:
+
+```bash
+SHOW ALL TABLES;
+```
+
+Execute a sample query:
+
+```sql
+SELECT * FROM metrics__dev.metrics_v0 LIMIT 5;
+```
+
+## Testing
+
+Compile against your local DuckDB copy by running:
+
+```bash
+sqlmesh plan dev
+```
+
+As this will run against everything in the dataset, you may want to pick a shorter date range (that you know has data in it), eg:
+
+```bash
+sqlmesh plan dev --start 2024-12-01 --end 2024-12-31
+```
+
+If a source that's in BigQuery is missing from DuckDB, check the `initialize_local_duckdb` function in [utils.py](warehouse/metrics_tools/local/utils.py). 
+You can add new models as `bq_to_duckdb` parameters, eg:
+
+```python
+"opensource-observer.oso_playground.YOUR_MODEL": "sources.YOUR_MODEL",
+```
+
+...and then reference the model in your sqlmesh code via `@oso_source('YOUR_MODEL')`.
+
+Important: whenever you add a new source, you will need to re-initialize your local database:
+
+```bash
+oso metrics local initialize
+```
+
+Then you can run to compile the latest models:
+
+```bash
+sqlmesh plan dev 
+```
+
+And if it executes successfully, view it in DuckDB:
+
+```sql
+SELECT * FROM metrics__dev.YOUR_MODEL LIMIT 5;
 ```
 
 ## Metrics Overview