Docs: Add MD for var-naming

src/ansiblelint/rules/var_naming.md

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+# var-naming
+
+This rule checks variable names to ensure they conform with requirements.
+
+Variable names must contain only lowercase alphanumeric characters and the underscore `_` character.
+Variable names must also start with either an alphabetic or underscore `_` character.
+
+For more information see the [creating valid variable names](https://docs.ansible.com/ansible/latest/user_guide/playbooks_variables.html#creating-valid-variable-names) topic in Ansible documentation.
+
+## Problematic Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  vars:
+    CamelCase: true # <- Contains a mix of lowercase and uppercase characters.
+    ALL_CAPS: bar # <- Contains only uppercase characters.
+    v@r!able: baz # <- Contains special characters.
+```
+
+## Correct Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  vars:
+    lowercase: true # <- Contains only lowercase characters.
+    no_caps: bar # <- Does not contains uppercase characters.
+    variable: baz # <- Does not contain special characters.
+```