added architecture diagram (#62)

ARCHITECTURE.md

@@ -0,0 +1,76 @@
+# General workflow
+
+```mermaid
+sequenceDiagram
+    actor User
+    participant repo as GitHub Repo
+    participant GH as GitHub Backend
+    participant App as Application
+    User->>repo: Creates/modifies <br>issue or comments
+    repo->>GH: Notifies backend
+    GH->>App: Sends Webhook
+    App->>App: Validates the webhook hash
+    App->>App: Validates webhook's trigger
+    App->>repo: Reads ".github/.jira_sync_config.yaml" file
+    App->>Jira: Searches for existing Jira issues
+    Jira->>App: Provides list of existing issues if any
+    App->>Jira: Creates/updates Jira issue
+    App->>repo: (Optional) Adds a comment on the Issue
+    App->>GH: Returns web response
+
+```
+
+# Issue created from template (with labels)
+
+This diagram illustrates a corner case scenario when issue is created from GitHub issue template that contains Labels.
+In this case GitHub sends two webhooks in parallel that causes issues in the asynchronous (stateless) service.
+
+```mermaid
+sequenceDiagram
+    actor User
+    participant repo as GitHub Repo
+    participant GH as GitHub Backend
+    participant App1 as Application (Unit 1)
+    participant App2 as Application (Unit 2)
+    User->>repo: Creates an issue with a label <br>applied directly from the template
+    repo->>GH: Notifies backend
+    par
+    GH->>App1: Sends Webhook (issue labeled)
+    GH->>App2: Sends Webhook (issue opened)
+    end
+
+    par
+    App1->>App1: Validates the webhook hash
+    App1->>App1: Validates webhook's trigger
+    App2->>App2: Validates the webhook hash
+    App2->>App2: Validates webhook's trigger
+    App1->>repo: Reads ".github/.jira_sync_config.yaml" file
+    App2->>repo: Reads ".github/.jira_sync_config.yaml" file
+    end
+    
+    par
+    App1->>Jira: Searches for existing Jira issues
+    App2->>Jira: Searches for existing Jira issues
+    end
+
+    par
+    Jira->>App1: Returns None, as there is no Jira issue
+    Jira->>App2: Returns None, as there is no Jira issue
+    end
+    
+    par
+    App1->>Jira: Creates/updates Jira issue
+    App2->>Jira: Creates/updates Jira issue
+    end
+
+    par
+    App1->>repo: (Optional) Adds a comment on the Issue
+    App2->>repo: (Optional) Adds a comment on the Issue
+    end
+    
+    par
+    App1->>GH: Returns web response
+    App2->>GH: Returns web response
+    end
+
+```