Added documentation

Change-Id: Iaa6bb0470652d3d0dc97c6320dbf210567ccec80
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>

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+# Configuration
+
+Configuration defines all in **Signal composer** each signals and sources has to be defined in it.
+At start, configuration will be searched in default binding configuration directory which should be
+_/var/lib/afm/applications/signal-composer/1.0/etc/_. Binding search for a file name as _init-daemon*.json_. Others files could be used to split sections and don't have 1 big fat definitions file.
+
+Saying that you have 4 sections to define:
+
+- **metadata**: Main parts and the only one that can't be in a separates configuration. This must appears in the main configuration file.
+- **plugins** (optionnal): Declare plugins that will be used by *sources* and *signals* for the subscription and composition.
+- **sources**: Declare **low level** signals sources (eg. low-can, gps, mraa, ...).
+- **signals**: Declare signals, virtuals and raw.
+
+## Metadata
+
+Here we define some metadata about **signal composer** binding. Fields to configure
+are :
+
+- **label**: self-explanatory
+- **version** (optionnal): self-explanatory
+- **api**: name that the binding will be initialized to and later be accessible by **Application Framework**
+- **info** (optionnal): self-explanatory
+- **require** (optionnal): list of required external apis.
+- **files** (optionnal): list of additionnals files. **ONLY NAME** or part of it without extension. These
+ files are loaded to find sections: *plugins*, *sources* and *signals*.
+
+## Plugins
+
+This section is the only which is optionnal, it is needed if you develop specifics C/C++ plugins
+to be used with signal-composer. LUA and API consumption does not need plugins.
+
+Default path to search for a plugin is in the binding library directory in a subdirectory _plugins_
+_/var/lib/afm/applications/signal-composer/1.0/lib/plugins_. Else you could use the environment variable _CONTROL_PLUGIN_PATH_ with a semicolon separated list of
+directory.
+
+Fields are:
+
+- **label**: Define the plugin name. This is that label that will be used on **sources** and
+ **signals** callbacks.
+- **version** (optionnal): self-explanatory
+- **info** (optionnal): self-explanatory
+- **basename**: shared library file name **without** the extension.
+
+## Sources
+
+A source is a **low level** API that will be consume by the **signal composer**
+to be able to expose those signals with additionnals treatments, filtering,
+thinning,... and create new virtuals signals composed with basic raw signals.
+
+A source is defined with following fields:
+
+- **api**: Name of the source API.
+- **info** (optionnal): self-explanatory
+- **init** (optionnal): an **action** to take to initialize a source. May you have  to call a verb from that API, of create a files etc.
+- **getSignals** (optionnal); an **action** to take to get signals from that
+ source. These callback will be used for each signals defined later in the **signals** section. Dedicated arguments for each signal could be defined in
+ **signals**.
+
+## Signals
+
+A signal definition could be either a **raw** one or a **virtual** one. A **virtual signal** is a set of existing **raw signals** associated to an **action**
+on reception which will compute the value of the signal.
+
+- **id**: Unique identifier used inside **signal composer**, used to compose virtual signals.
+- **event**: specify a **raw signal** coming from **low level** sources. Couldn't
+ be used with **depends** field, only one of them is possible.
+- **depends**: specify others signals **id** that compose it (eg: heading is
+ composed with longitude+latitude signals.). Couldn't be used with **event** field
+ at same time.
+- **unit** (optionnal): Unit used to exprime the signal
+- **frequency** (optionnal): Frequency maximum at which the signal could be
+ requested or sent. This is a thinning made at **high level** so not best suited for performance. Used **low level** native filtering capabilities when possible.
+- **getSignalsArgs**: a JSON object used at subscription time. Meant to enabled
+ filtering capabilities at subscription and to be able to customize in general a subcription request by signal if needed.
+- **onReceived**: an **action** to take when this signal is received!