Reword warning on default values and add documentation building tip t…

…o README

README.md

@@ -111,6 +111,11 @@ sbt makeSite
 
 HTML Documentation should be generated at `target/sphinx/html/index.html`.
 
+Alternatively use Docker:
+
+```bash
+docker run --rm -v ./docs:/docs sphinxdoc/sphinx:3.2.1 bash -c "pip install sphinx-rtd-theme && make html"
+```
 
 ## Thanks
 

docs/source/transformers/default-values.rst

@@ -9,8 +9,8 @@ Enabling default values in generated transformer
 
 .. warning::
 
-  Before 0.7.0 default values was enabled by default. Due to feedback it was
-  changed to disabled by default for safety.
+  Prior to version 0.7.0 fallback to default values was automatically enabled
+  and required explicit disabling.
 
 Field's default value can be enabled as a target value when constructing
 target object. The support for them has to be explicitly enabled to avoid