[MAP] Adds fallback to original value when input not found (#13560)

* [MAP] Adding fallback to original value
Solves #10092

Signed-off-by: clinique <gael@lhopital.org>

bundles/org.openhab.transform.map/README.md

@@ -7,7 +7,9 @@ This file should be in property syntax, i.e. simple lines with "key=value" pairs
 The file format is documented [here](https://docs.oracle.com/javase/8/docs/api/java/util/Properties.html#load-java.io.Reader-).
 To organize the various transformations one might use subfolders.
 
-A default value can be provided if no matching entry is found by using "=value" syntax
+A default value can be provided if no matching entry is found by using "=value" syntax. 
+Defining this default value using `_source_` would then return the non transformed input string.
+
 
 ## Example
 
@@ -31,6 +33,7 @@ white\ space=using escape
 | `white space` | `using escape` |
 | `anything`    | `default`      |
 
+
 ## Usage as a Profile
 
 The functionality of this `TransformationService` can be used in a `Profile` on an `ItemChannelLink` too.