adding doc about pagination

source/static/puml/pagination-sequence.puml

@@ -0,0 +1,20 @@
+@startuml
+
+actor "controller, service, ..." as x
+participant "paginator"
+participant "Request"
+
+x -> paginator: getCurrentPage()
+activate paginator
+
+paginator -> Request: read the `page` parameter in GET request
+activate Request
+Request -> paginator
+deactivate Request
+
+paginator -> paginator: construct a page object for current page number
+paginator -> x: return the `page`
+deactivate paginator
+
+@enduml 
+