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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+# AGL Advanced Audio Architecture (4a)
+
+## Abstract
+
+4a is an audio framework made to handle automotive use cases like stream
+priority and interruptions. It does **not** deal with the audio by itself but
+rather with the policies and signaling around it. This means that 4a must be
+used in conjunction with an other API that is in charge of the audio stuff,
+like **alsa** or **pulseaudio**.
+
+## Architecture
+
+### Global architecture
+
+![applications architecture diagram](images/apps-architecture.svg)
+
+This diagram shows the global architecture that any application playing audio
+should follow. As a developer, when you want to play audio, you rely on APIs
+such as alsa, pulseaudio or higher level API(s). To play audio you usually have
+some kind of device URI that you have to open in order to write audio data to
+it.
+
+4a does **not** change anything about that. You still have to do all of this. 4a
+provides permissions, signals and policies. This means that all compliant
+applications have to ask 4a for a 'role' when they want to play audio. If 4a
+grants them the permission, it returns a device URI that applications have to
+open.
+
+Nothing prevent an application to directly open a device, but in this case no
+policies can be applied and access permissions are handled by system policies.
+
+### 4a Architecture
+
+![4a's architecture diagram](images/4a-architecture.svg)
+
+Even if applications only deal with the high level API, 4a is made of multiple
+components that rely on each other.
+
+The high level API uses the hal-manager to list enabled HALs then it uses
+directly those HALs.
+
+HALs rely directly on drivers and/or softmixer.