Add model documentation

docs/models.md

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+## Table of Contents
+### Palladio Component Model (PCM)
+1. [Bank Branches](#bank-branches)
+2. [Branching Online Shop](#branching-online-shop)
+3. [CoCar](#cocar)
+4. [Corona Warn App](#corona-warn-app-cwa)
+5. [International Online Shop](#international-online-shop)
+6. [Mobility as a Service(MaaS) Ticket System](#mobility-as-a-servicemaas-ticket-system)
+7. [Travel Planner](#travel-planner)
+
+### Data Flow Diagrams (DFDs)
+1. Online Shop 
+2. Simple Online Shop 
+3. Models from TU Hamburg
+
+
+
+# Models
+## PCM Models 
+### Bank Branches
+#### Overview
+This is an scenario from [Seifermann et al.](https://doi.org/10.1016/j.jss.2021.111138).
+It contains a banking system deployed in the US and Asia.
+Two actors interact with the system:
+A clerk that can register customers, find customers and determine a credit line for them. 
+A manager can register celebrities or move customers between regions, in addition to the capabilities of a clerk. 
+
+#### Characteristic Types
+There are 4 characteristic types in the model: 
+The `Role` characteristic type describes the role of an actor, and can be either `Clerk` or `Manager`.
+The `Location` characteristic type describes the location of an actor, and can be either `USA` or `Asia`.
+The `Origin` characteristic type describes the origin of the request, and can be either `USA` or `Asia` as well. 
+The `Status` characteristic type describes the status of the customer. It can be either `Regular` or `Celebrity`.
+
+#### Security Requirements
+The two security requirements for this model are the following:
+1. Clerks must only access data about customers in the same region 
+2. Only managers should be able to access data about celebrities
+
+### Branching Online Shop 
+#### Overview 
+This is an 
+
+### CoCar 
+
+### Corona Warn App (CWA)
+
+### International Online Shop 
+
+### Mobility as a Service(MaaS) Ticket System 
+
+### Travel Planner
+#### Overview 
+This is an scenario from [Seifermann et al.](https://doi.org/10.1109/ICSA.2019.00009).
+It models a user booking a flight with an travel planner application.
+Flights are queried using a travel agency that in turn communicates with the airline to determine flights. 
+Using the flight information the user books an flight with their credit card details. 
+
+#### Characteristic Types 
+There are 2 characteristic types in the model: 
+The `AssignedRoles` characteristic type describes the role of an system component. It can be either `User` or `Airline`.
+The `GrantedRoles` characteristic type describes the roles that can access given data. It can be either `User` or `Airline`.