Add snippet and documentation

.vscode/C4.code-snippets

@@ -564,5 +564,16 @@
 			"LAYOUT_AS_SKETCH()"
 		],
 		"description": "Sketch layout for C4 diagram."
-	}
+	},
+	"C4_Auto_Number_Relationships": {
+		"scope": "plantuml",
+		"prefix": [
+			"Autonumber relationships",
+			"Add relationship numbers"
+		],
+		"body": [
+			"AUTONUMBER_RELATIONSHIPS()"
+		],
+		"description": "Automatically add relationship numbers to C4 diagram."
+	},
 }

LayoutOptions.md

@@ -139,4 +139,26 @@ Rel(web_app, twitter, "Gets tweets from", "HTTPS")
 
 ![HIDE_STEREOTYPE Sample](https://www.plantuml.com/plantuml/png/NL1DIyD05BplhrZheIdKX8edJuqrMC5gQ5B5KzWcJxkmN-o-DHJnl_j2gnNtCl1ctfkPdGSK7gDMV7b_MpHLNQoBf_grB7Wbj5F0pgHfLUo0xn1TkCuoB_hqNU8kRcC0trg3O31jhSv4vwKm7ogwc2skBaeb36vM3vaI205fr2n8BQG1dpgoPEoVPp9Xh0GVh4b4fwMPyb4-e0pe1Le_ch1g7n1qSsEZwOJV9-rioiU7gEC9_sIStgzBqiEs4SxHdHVOyjmv9Lk1qfeHZHRdKvvNPVfOrBJYWL-cCxvwNWnShfJj0nyQ1ewMKpqdIAAZGabUlEub8dYEKeCYhq3Plexf4ZoGTESe8vAjGVsqEiNQ1DeaNDPrMP5g4FsofpDtW1IZXm7UlDL_L-PWElhCRm00 "HIDE_STEREOTYPE Sample")
 
+## AUTONUMBER_RELATIONSHIPS()
 
+Automatically prefix your relationships with an auto-incremented index.  Useful for complex sequences of events, callbacks, and other unclear relationships
+
+```csharp
+@startuml AUTONUMBER_RELATIONSHIPS Sample
+!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml
+
+HIDE_STEREOTYPE()
+AUTONUMBER_RELATIONSHIPS()
+
+Person(admin, "Administrator")
+System_Boundary(c1, 'Sample') {
+    Container(web_app, "Web Application", "C#, ASP.NET Core 2.1 MVC", "Allows users to compare multiple Twitter timelines")
+    ContainerDb(db, "Database", "SQL Server", "Stores comparison info")
+}
+System(twitter, "Twitter")
+
+Rel(admin, web_app, "Uses", "HTTPS")
+Rel(web_app, twitter, "Gets tweets from", "HTTPS")
+Rel_Right(web_app, db, "Stores", "SQL")
+@enduml
+```

README.md

@@ -171,6 +171,7 @@ C4-PlantUML also comes with some layout options to make it easy and reuseable to
 * [LAYOUT_WITH_LEGEND()](LayoutOptions.md#layout_with_legend)
 * [LAYOUT_AS_SKETCH()](LayoutOptions.md#layout_as_sketch)
 * [HIDE_STEREOTYPE()](LayoutOptions.md#hide_stereotype)
+* [AUTONUMBER_RELATIONSHIPS()](LayoutOptions.md#autonumber_relationships)
 
 ## Snippets for Visual Studio Code