adds README for Airtable source

sources/airtable/README.md

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+---
+title: Airtable
+description: dlt source for Airtable.com API
+keywords: [airtable api, airtable source, airtable, base]
+---
+
+
+# Airtable
+
+[Airtable](https://airtable.com/) is a spreadsheet-database hybrid, with the features of a database but applied to a spreadsheet. It is also marketed as a low‒code platform to build next‒gen apps. Spreadsheets are called *airtables* and are grouped into *bases*.
+
+This dlt source creates a dlt resource for every airtable and loads it. 
+
+To load airtables:
+
+1. identify the *base* you want to load from. The ID starts with "app". See [how to find airtable IDs](https://support.airtable.com/docs/finding-airtable-ids)
+2. identify the *airtables* you want to load. You can identify in three ways:
+
+| filter             | Description                                              |
+| ---------------- | -----------------------------------------------------------|
+| table_names  | retrieves *airtables* from a given *base* for a list of user-defined mutable names of tables |
+| table_ids    | retrieves *airtables* from a given *base* for a list of immutable IDs defined by airtable.com at the creation time of the *airtable*. IDs start with "tbl". See [how to find airtable IDs](https://support.airtable.com/docs/finding-airtable-ids) |
+| empty filter | retrieves all *airtables* from a given *base* |
+
+
+## Supported write dispositions
+This connector supports the write disposition `replace` and not `append` because 
+a *base* can contain only [up to 50k records in the most expensive plan](https://support.airtable.com/docs/airtable-plans)
+
+If resource consumption for data loading becomes a concern in practice request the `append` loading method.
+
+
+## Initialize the pipeline
+
+```bash
+dlt init airtable duckdb
+```
+
+Here, we chose duckdb as the destination. Alternatively, you can also choose redshift, bigquery, or
+any of the other [destinations](https://dlthub.com/docs/dlt-ecosystem/destinations/).
+
+
+## Add credentials
+
+1. [Obtain a Personal Access Token](https://support.airtable.com/docs/creating-and-using-api-keys-and-access-tokens).
+  If you're on an enterprise plan you can create an [Airtable Service Account](https://dlthub.com/docs/dlt-ecosystem/verified-sources/chess).
+  Place the key into your `secrets.toml` like this:
+```
+[sources.airtable]
+access_token = "pat***"
+```
+2. Follow the instructions in the [destinations](https://dlthub.com/docs/dlt-ecosystem/destinations/) 
+  document to add credentials for your chosen destination.
+
+
+## Run the pipeline
+
+1. Install the necessary dependencies by running the following command:
+
+   ```bash
+   pip install -r requirements.txt
+   ```
+
+2. Now the pipeline can be run by using the command:
+
+   ```bash
+   python airtable_pipeline.py
+   ```
+
+3. To make sure that everything is loaded as expected, use the command:
+
+   ```bash
+   dlt pipeline airtable_pipeline show
+   ```
+
+💡 To explore additional customizations for this pipeline, we recommend referring to the example pipelines in `airtable_pipeline.py`.