added file name suffix principle for data

Author: sprankhub

docs/data.md

@@ -121,6 +121,7 @@ The following conventions apply to MFTF `<data>`:
 * A `<data>` file may contain multiple data entities.
 * Camel case is used for `<data>` elements. The name represents the `<data>` type. For example, a file with customer data is `CustomerData.xml`. A file for simple product would be `SimpleProductData.xml`.
 * Camel case is used for the entity name.
+* The file name must have the suffix `Data.xml`.
 
 ## Example
 
@@ -263,4 +264,4 @@ Attributes|Type|Use|Description
 [Actions]: ./test/actions.md
 [category creation]: http://docs.magento.com/m2/ce/user_guide/catalog/category-create.html
 [Credentials]: ./credentials.md
-[test actions]: ./test/actions.md#actions-returning-a-variable
+[test actions]: ./test/actions.md#actions-returning-a-variable