[GH-24] Add understandable documentation for non-developers

docs/Document.md

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+ # `cardano-shell` overview
+
+`cardano-shell` is an thin layer which brings all the other modules working
+ together and makes sure that they have the required resources 
+ (configuration, exception handling, monitoring, logging, ...).
+
+The shell is also responsible for:
+
+* Logging
+* Monitoring
+* Configuration
+* Exception handling
+* Launcher
+
+## Features
+
+Various **features** are required for the cardano node to operate.
+
+* Blockchain layer
+* Ledger layer
+* Logging layer
+* Monitoring layer
+* Network layer
+* Wallet backend layer
+
+The shell will act as a **glue** of these features. It provides required
+ resources/configurations to each of these features as well as resolving its dependencies.
+
+![shell-diagram](https://user-images.githubusercontent.com/6264437/47286815-88df4100-d5f0-11e8-92a7-c807b6d3b47a.jpg)
+
+## Resolving dependencies between the features
+
+There are **dependencies** between the features meaning some of the features
+ depend on others. For example,
+
+* `networking` feature requires `logging` and `monitoring`
+* `blockchain` feature `logging` and `networking`
+
+and so on.
+
+The shell will resolve these dependencies by having each of the features to
+ produce a `layer` and distributing them to other features that depend on it.
+
+### Layer
+
+To put it simple, **a layer is a record of functions that each feature generates when initialized**. 
+For example, logging feature will produce logging layer when initialized.
+The logging layer will have a list of functions related to logging such as
+`logInfo`, `logDebug` which the other features can use.
+
+![layer](https://user-images.githubusercontent.com/15665039/55375129-e1bee800-5545-11e9-82c0-f7bef87deaf3.jpg)
+
+Dependencies between the features are resolved by passing these layers. 
+
+- If `networking` feature requires `logging`, then it will have logging
+layer as a dependency when initializing.
+
+![network](https://user-images.githubusercontent.com/15665039/55375015-7f65e780-5545-11e9-9fd8-ca2d37d7cc28.jpg)
+
+- If `blockchain` feature requires `logging` and `network`, then shell will
+provide those layers as dependencies.
+
+![blockchain](https://user-images.githubusercontent.com/15665039/55375281-8a6d4780-5546-11e9-9240-21d9ca8cbc46.jpg)
+
+Layers are defined in such a way that it can be **stubbed**. This will 
+allow the developer to write test cases on each features with ease.
+