updated doc (see #277)

docs/md/entity.md

@@ -24,6 +24,7 @@
     * [Archives](#archives)
     * [One example to rule them all](#one-example-to-rule-them-all)
   * [Prototype](#prototype)
+  * [The actor class](#the-actor-class)
   * [Helpers](#helpers)
     * [Dependency function](#dependency-function)
     * [Tags](#tags)
@@ -941,6 +942,25 @@ Furthermore, the codebase may be easier to maintain, since updating a prototype
 is much less error prone than jumping around in the codebase to update all the
 snippets copied and pasted around to initialize entities and components.
 
+## The actor class
+
+The `actor` class is designed for those who don't feel immediately comfortable
+working with components or for those who are migrating a project and want to
+approach it one step at a time.
+
+This class acts as a thin wrapper for an entity and for all its components. It's
+constructed with a registry to be used behind the scenes and is in charge of the
+destruction of the entity when it goes out of the scope.<br/>
+An actor offers all the functionalities required to work with components, such
+as the `assign` and` remove` member functions, but also `has`,` get`, `try_get`
+and so on.
+
+My advice isn't to use the `actor` class to hide entities and components behind
+a more object-oriented interface. Instead, users should rely on it only where
+strictly necessary. In all other cases, it's highly advisable to become familiar
+with the model of `EnTT` and work directly with the registry, the views and the
+groups, rather than with a tool that could introduce a performance degradation.
+
 ## Helpers
 
 The so called _helpers_ are small classes and functions mainly designed to offer