Create GraphClientsDesign.md

Author: nikithauc

design/GraphClientsDesign.md

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+
+### Graph JS Deliverables:
+
+Note: Package names yet to be decided
+
+1. `microsoftgraph/microsoft-graph-javascript-service`: 
+
+    - Generated library which the request builders. Generated using Kiota builder.
+
+    - Will have dependencies on :
+        - @microsoftgraph/microsoft-graph-javascript-core
+        - @microsoft/kiota-abstractions
+        - @microsoft/kiota-http-fetchlibrary
+        - @microsoft/kiota-serialization-json
+        
+2. `microsoftgraph/microsoft-graph-javascript-core`:
+
+    - Contains Graph customizations and tasks such as PageIteration, LargeFileUpload.
+
+    - Will have dependencies on :
+      - @microsoft/kiota-abstractions
+      - @microsoft/kiota-http-fetchlibrary
+        
+### Usage of the two libraries :
+
+As mentioned in PR: #558
+
+Goals: 
+
+- A Graph JS SDK user should not be required to create separate client instances for Graph Service library or the Graph Core library.
+- To achieve this, the `Graph Service Client` should also expose the features of `Graph Core Client`.
+
+Design:
+
+1. `Graph Service client` should extend from `Graph Core Client`:
+
+```
+
+import { Client as GraphCoreClient } from `@microsoft/microsoft-graph-javascript-core`;
+
+
+class GraphServiceClient extends GraphCoreClient {
+
+    public api(){
+        super.api();
+    }
+}
+```
+
+- To acheive the above design we will need to customize the auto-generated `GraphServiceClient`.
+
+