Add doc for adopting this package

README.md

@@ -12,3 +12,6 @@ This extension pack includes the following features
 - [Primitive domains](./docs/primitive-domains.md)
 - [A common criteria based filter](./docs/criteria-based-filter.md) for Domains and Selector
 
+## Documentation
+
+- [How to adapt](./docs/how-to-adapt.md) when coming from fflib-apex-common 

docs/how-to-adapt.md

@@ -0,0 +1,8 @@
+# fflib-apex-extensions
+
+## How to adapt
+When you are familiar with using fflib-apex-common, then you need to be aware of a few differences that you will encounter when adopting this extension package.
+
+### Changes to the domain
+A Domain class is extended from the domain class `SObjects` instead of `fflib_SObjects`.
+