Updated documentation

Updated documentation up to date and using the gitbook format.

Change-Id: Ie5d6a3bb489b9a9a95e21f5edd05bdd77bff5816
Signed-off-by: Loïc Collignon <loic.collignon@iot.bzh>

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+# Components
+
+4a framework is made of several components which provide features to each other.
+The High Level API component is the only one that exports a public API. This
+means that, as an application developer you only need to care about the High
+Level API.
+
+## High level API
+
+This is the publicly exported API. It exposes one verb for each audio roles,
+plus **get_roles** to get the list of audio roles.
+
+This component manages audio roles and applies permissions, signaling and
+policies. Because each role is a unique verb, it means that each role can have
+its own smack label.
+
+It makes use of the **4a-hal-manager** to list all HALs and their
+configurations. Then it uses directly the enabled HALs.
+
+## 4a-hal-manager
+
+This component is responsible for HALs detection, initialization and state
+management.
+
+## 4a-hal-*
+
+This block represents all the different HALs. You can load multiple HALs
+provided that they don't interfere with each other.
+HALs can be provided by two means:
+
+* Built directly by the **4a-hal-manager** using it's controller and json files.
+* External HAL (like the unicens one), that have to register themselves to the
+  **4a-hal-manager**.
+
+HALs manage audio streams and zones. The streams are bound to audio roles.
+
+## 4a-softmixer
+
+This component abstracts the real devices to virtual ones and provides some
+basic mixing capabilities for devices that don't have this feature builtin.
+It exposes streams named like the audio roles.
+
+This component can be used by HALs, but this is not mandatory.
+
+HALs can either :
+
+* use 4a-softmixer (for example when hardware doesn't provide mixing feature),
+  or
+* take advantage from the hardware to provide the mixing capabilities.