[DOC] update the documentation to new config and env

part of #306

doc/reference/config.md

@@ -0,0 +1,56 @@
+# 🦉 Config 🦉
+
+The Owl framework is designed to work in many situations.  However, it is
+sometimes necessary to customize some behaviour.  This is done by using the
+global `config` object. It currently has two keys:
+
+- [`mode`](#mode),
+- [`env`](#env).
+
+## Mode
+
+By default, Owl is in _production_ mode, this means that it will try to do its
+job fast, and skip some expensive operations. However, it is sometimes necessary
+to have better information on what is going on, this is the purpose
+of the `dev` mode.
+
+Owl has a mode flag, in `owl.config.mode`. Its default value is `prod`, but
+it can be set to `dev`:
+
+```js
+owl.config.mode = "dev";
+```
+
+Note that templates compiled with the `prod` settings will not be recompiled.
+So, changing this setting is best done at startup.
+
+An important job done by the `dev` mode is to validate props for each component
+creation and update. Also, extra props will cause an error.
+
+## Env
+
+An Owl application needs an [environment](environment.md) to be executed. The
+environment has an important key: the [QWeb](qweb.md) instance, which will render
+all templates.
+
+Whenever a root component is mounted, Owl will take the environment from
+`owl.config.env` and use it to setup the component (and its children).
+
+- if no environment was setup, an empty environment will be generated,
+- if an environment exists, but does not have a QWeb key, a new QWeb instance
+  will then be added to the environment.
+
+The correct way to customize an environment is to simply modify `owl.config.env`
+before the first component is created:
+
+```js
+owl.config.env = {
+    _t: myTranslateFunction,
+    user: {...},
+    services: {
+        ...
+    },
+};
+const app = new App();
+app.mount(document.body);
+```