feat(doc): added EachKey documentation

Author: andrepimpao

docs/03-rules.md

@@ -30,4 +30,5 @@
 
 ## Iterable Rules
 
-- [EachValue](03x-rules-eachvalue.md)
+- [EachValue](03x-rules-each-value.md)
+- [EachKey](03x-rules-each-key.md)

docs/03x-rules-each-key.md

@@ -0,0 +1,49 @@
+## EachKey
+
+Validates every key of an `array` or object implementing `\Traversable` with a given set of rules.
+
+```php
+EachKey(
+    Validator $validator,
+    string $message = 'Invalid key: {{ message }}'
+);
+```
+
+## Basic Usage
+
+```php
+Validator::eachKey(Validator::notBlank()->type('string'))->validate(['red' => '#f00', 'green' => '#0f0']); // true
+Validator::eachKey(Validator::notBlank()->type('string'))->validate(['red' => '#f00', 1 => '#0f0']); // false
+```
+
+> **Note**
+> An `UnexpectedValueException` will be thrown when the input value is not an `array` or an object implementing `\Traversable`.
+
+## Options
+
+### `validator`
+
+type: `Validator` `required`
+
+Validator that will validate each key of an `array` or object implementing `\Traversable`.
+
+### `message`
+
+type: `string` default: `Invalid key: {{ message }}`
+
+Message that will be shown if at least one input value key is invalid according to the given `validator`.
+
+```php
+Validator::eachKey(Validator::notBlank())->assert(['red' => '#f00', 1 => '#0f0'], 'Test'); 
+// Throws: Invalid key: The "Test" key should be of type "string", "1" given.
+```
+
+The following parameters are available:
+
+| Parameter       | Description                                      |
+|-----------------|--------------------------------------------------|
+| `{{ value }}`   | The current invalid value                        |
+| `{{ name }}`    | Name of the invalid value                        |
+| `{{ key }}`     | The key of the invalid iterable element          |
+| `{{ element }}` | The value of the invalid iterable element        |
+| `{{ message }}` | The rule message of the invalid iterable element |