Add README for Postman collection

postman/README.md

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+# Camunda Postman Collection
+This Postman collection contains API calls with sample payloads for both general Camunda calls and FOLIO process specific calls. It can be imported directly into Postman.
+
+## Camunda
+Details about Camunda specific calls
+* **Get Tasks** 
+    * Get list of all open tasks. Can also pass in filters (no filters currently used in Postman collection)
+    * [https://docs.camunda.org/manual/7.9/reference/rest/task/get-query/](https://docs.camunda.org/manual/7.9/reference/rest/task/get-query/)
+* **Get Tasks Filtered**
+    * Get filtered list of tasks, slightly more powerful than above query, allows for filtering by multiple process or task variables
+    * Current Postman sample filters by process, task name, and tenantId
+    * [https://docs.camunda.org/manual/7.9/reference/rest/task/post-query/](https://docs.camunda.org/manual/7.9/reference/rest/task/post-query/) 
+* **Complete Task by Id**
+    * Completes task and updates process variables
+    * Current Postman sample has variables required to make an Okapi request/response
+    * [https://docs.camunda.org/manual/7.9/reference/rest/task/post-complete/](https://docs.camunda.org/manual/7.9/reference/rest/task/post-complete/)
+
+
+## Claim Returned Process
+* **Start**
+    * This message starts the Claim Returned Process with a set of variables
+        * `businessKey` - a unique identifier for the process instance
+        * `tenantId`
+        * `processVariables` - currently dummy data, will update with real world data when we have integrations to FOLIO and Okapi
+* **Update Claim Message**
+    * This message can be sent to update the claim with a `claimAction`
+    * Note this can be completed as a task from the tasklist as well. Sending this payload will interrupt the task and cancel it. 
+* **External Update Message**
+    * This message interrupts the process that a claim has been updated from an external source and terminates the process
+    * The message correlates to the `messageName` (specific to this process), the `businessKey` (unique identifier of the process instance), and the `tenantId`
+
+## Purchase Request Process
+* **Start**
+    * This message starts the Purchase Request Process with a set of variables
+       * `tenantId`
+       * `processVariables` - currently dummy data, will update with real world data when we have integrations to FOLIO and Okapi
+* **Receive Message**
+    * This message is meant to simulate an order being received
+    * Once an order for the request has been created, the process waits for this message to be sent in order to advance the token
+    * The message is correlated by the `bookId` and `orderId`