Add text entity developer docs (#1536)

docs/core/entity/text.md

@@ -0,0 +1,38 @@
+---
+title: Text Entity
+sidebar_label: Text
+---
+
+A text entity is an entity that allows the user to input a text value to an integration. Derive entity platforms from [`homeassistant.components.text.TextEntity`](https://github.com/home-assistant/home-assistant/blob/master/homeassistant/components/text/__init__.py)
+
+## Properties
+
+:::tip
+Properties should always only return information from memory and not do I/O (like network requests). Implement `update()` or `async_update()` to fetch data or build a mechanism to push state updates to the entity class instance.
+:::
+
+| Name | Type | Default | Description
+| ---- | ---- | ------- | -----------
+| mode | string | `text` | Defines how the text should be displayed in the UI. Can be `text` or `password`.
+| native_max | int | 100 | The maximum number of characters in the text value (inclusive).
+| native_min | int | 0 | The minimum number of characters in the text value (inclusive).
+| pattern | str | `None` | A regex pattern that the text value must match to be valid.
+| native_value | str | **Required** | The value of the text.
+
+Other properties that are common to all entities such as `icon`, `name` etc are also applicable.
+
+
+## Methods
+
+### Set value
+
+```python
+class MyTextEntity(TextEntity):
+    # Implement one of these methods.
+
+    def set_value(self, value: str) -> None:
+        """Set the text value."""
+
+    async def async_set_value(self, value: str) -> None:
+        """Set the text value."""
+```