Add documentation for to_var function #282

Author: TobiasNx

README.md

@@ -267,6 +267,23 @@ put_vars(
 
 [Java Code](https://github.com/search?type=code&q=repo:metafacture/metafacture-fix+path:FixMethod.java+"+put_vars+{")
 
+##### `to_var`
+
+Defines a single global variable that can be referenced with `$[<variableName>]` and assinges the value of the `<sourceField>`.
+
+```perl
+to_var("<sourceField>", "<variableValue>")
+```
+
+Options:
+
+- `"default"`: provide default value if source field does not exist.
+
+
+[Example in Playground](https://metafacture.org/playground/?example=to_var)
+
+[Java Code](https://github.com/search?type=code&q=repo:metafacture/metafacture-fix+path:FixMethod.java+"+to_var+{")
+
 #### Record-level functions
 
 ##### `add_field`