Added a small bit of clarification for indexing

series/Readme.md

@@ -14,9 +14,10 @@ or, in Markdown syntax
 The plugin collects all articles belonging to the same series and provides
 series-related variables that you can use in your template.
 
+#### Indexing
 By default articles in a series are ordered by date and then automatically numbered.
 
-If you want to force a given order just specify the `:series_index` metadata,
+If you want to force a given order just specify the `:series_index:` metadata or in Markdown `series_index:`,
 starting from 1. All articles with this enforced index are put at the beginning of
 the series and ordered according to the index itself. All the remaining articles
 come after them, ordered by date.