EOS-12894: Created High Level Overview document

Added description for NSAL Components
	- KVtree
	- KVnode
	- Xattr
	- Tenant
	- Namespace
	- KVstore

Moved high level overview doc inside doc folder

Added cortx-nsal arcitecture diagram inside image folder

Added architectural diag link to overview doc

Signed-off-by: Shreya Karmakar <shreya.karmakar@seagate.com>

README.md

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 [![Codacy Badge](https://app.codacy.com/project/badge/Grade/f0d0abc1e7534ce68c7d4fb4787ded9c)](https://www.codacy.com?utm_source=github.com&utm_medium=referral&utm_content=Seagate/cortx-nsal&utm_campaign=Badge_Grade)
 
 # NSAL
-Namespace Abstraction Layer is created to facilitate namespace
-management on top of cloud object storage.
+Namespace Abstraction Layer is created to facilitate the namespace management and abstraction on top of a object/cloud storage.
 
 ### Disclaimer
 Please refer to the shared disclaimer (https://github.com/Seagate/cortx-posix#disclaimer) about this code repository.
+
+## Components
+
+The Namespace abstraction layer is implemented using the following entities/components
+- Extended attributes (Xattrs)
+- KVtree
+- KVnode
+- Tenants
+- Namespace
+- KVstore
+
+Refer to the [CORTX-NSAL High Level Overview](doc/HighLevelOverview.md) for a high level overview over each component. 

doc/HighLevelOverview.md

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+# High Level Overview
+
+## Acrhitectural overview
+![CORTX-NSAL Architecture](../images/cortx-nsal-arch.jpg)
+
+## KVstore
+
+KVstore gives capability of basic key-value storing.
+
+KVstore acts as an entry point for all the components of NSAL to communicate with the cortx-motr APIs. The kvstore APIs will provide the basic interfaces. i.e. SET, GET, DELETE of key-value entries at the backend.  
+
+Supported storage-plugin by kvstore-
+ - cortx-motr
+
+## KVtree
+
+KVtree is a logical tree representation which can be used to store heirarchical data.
+It is an in-memory object that can be created in a given namespace and works on a particular index created by namespace.
+A kvtree always has at least one node (root) whose attributes are stored on disk.
+
+Each KVtree object contains a list of `KVnodes`. Each kvnode present in the kvtree can have multiple children.
+
+Note: Every kvtree has a namespace.
+
+## KVnode
+
+KVnode is an in-memory representation of a node. 
+Every kvnode has a node ID which identifies it uniquely in the kvtree.
+KVnode has two types of attributes:
+1. Basic attribute 
+2. System attribute
+
+Both of these attributes are stored on-disk.
+Basic attributes act as the "core" of a kvnode, without which it cannot exist.
+System attributes are optional to the kvnode.
+
+### Use-case: 
+
+In a filesytem, 
+- The "/" root directory can act as the root kvnode of a kvtree. 
+- A file/directory/symlink can act as a kvnode.
+- Metadata of a file i.e the stats can be the basic attribute of that particular kvnode.
+- A kvnode which represents a symlink can use system attributes of that node to store symlink path.
+
+## Extended Attributes (Xattrs)
+
+Extended attributes are name:value pairs, where attribute names are `null-terminated strings`.
+
+Extended attributes are accessed as atomic objects. Supported xattr operations are SET, GET, DELETE and LIST.
+
+### Use-case:
+Xattrs can be used to provide additional functionality to a filesystem—
+- Additional security features such as Access Control Lists (ACLs)
+- Storing arbitrary additional information such as the mime type, character set or encoding of a file.
+
+Note: Xattrs are independent of KVTree/KVnode component.
+
+## Namespace
+
+Namespace acts as a `container` for a particular kvtree. All the data for a kvtree and its kvnodes are stored in this particular namespace (which creates an index).
+
+For creating a kvtree, namespace has to be created. The index created by the namespace is then used by the kvtree.
+
+All the namespaces(indexes) created are stored in a `global ns-meta index` in form of key-value pair.
+
+## Tenant
+
+Tenants are used to store export data. Tenant objects are stored in the same `global ns-meta index`. Every tenant object is uniquely identified via `tenant id` which is same as Export id.