[GH-24] Add understandable documentation for non-developers (#81)

* Initial write up for the document for non-developers

* Fix grammar

* Fix typo

* Add diagrams

* Replace interface with layer

* Add more description

* Add diagram

Author: HirotoShioi

docs/Document.md

@@ -0,0 +1,76 @@
+# 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, ...).
+
+## Cardano-node to operate as an peer
+
+For cardano-node to operate as an peer, various **features** are required:
+
+* Logging, to let the user know what's going on
+* Block validation
+* Managing blockchain/stake
+* Communicating with other peers
+* Submitting transaction
+* Providing interface/API so that the user can interact with the node
+
+and so on. Because of this, IOHK currently has multiple development teams, 
+each of which is responsible for developing specific features.
+
+* [blockchain layer](https://github.com/input-output-hk/cardano-ledger)
+* [ledger layer](https://github.com/input-output-hk/cardano-ledger)
+* [logging/monitoring layer](https://github.com/input-output-hk/iohk-monitoring-framework)
+* [network layer](https://github.com/input-output-hk/ouroboros-network)
+* [wallet backend layer](https://github.com/input-output-hk/cardano-wallet)
+
+### Bringing features together
+
+The thing to note is that all these features are being worked on **seperate**
+repository. 
+
+![scattered-modules](https://user-images.githubusercontent.com/15665039/55607001-dbcf3e00-57b5-11e9-89bf-9ed403c4e8e6.jpg)
+
+We need these modules working together to act as a node. This is where 
+`cardano-shell` comes in. The shell will act as a **glue** of these features,
+providing 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.