[docs] Add diagrams to docs

docs/diagrams/README.md

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+# Diagrams
+
+This folder contains diagrams about functionalities of CWL Viewer.
+The source file for the diagrams is stored in a SVG file. We suggest
+contributors to use Inkscape to edit the SVG file.
+
+## Flowcharts
+
+When a user submits a workflow to the CWL Viewer, the Spring controller
+will handle the request delegating most of the work to Spring services.
+
+![flow part 1](cwlviewer_flowchat_01.png)
+
+The first part of the flow is responsible for validating the request and
+checking if the workflow exists in the Postgres database, and if not
+creating a `QueuedWorkflow` in the database.
+
+![flow part 2](cwlviewer_flowchat_02.png)
+
+An asynchronous task is then scheduled to parse the CWL workflow with
+`cwltool`, use Apache Jena to create the workflow graph, and to store the
+`Workflow` object (not queued anymore) in the Postgres database.
+
+![flow part 3](cwlviewer_flowchat_03.png)
+
+This final part of the flow is also an asynchronous task, responsible for
+creating the Research Objects bundle file. It uses Apache Taverna API
+to create a `Bundle`, and stores parts of the Git repository, graph
+images generated by the `cwltool` and Jena graph data, and extra data
+for provenance. The result is updated in the `Workflow` object in Postgres.