go.talks/present: document the .extensions
Related: remove stale mention of -template flag.

R=golang-dev, campoy, adg
CC=golang-dev
https://golang.org/cl/6851046
diff --git a/present/doc.go b/present/doc.go
index bf43521..b9aae52 100644
--- a/present/doc.go
+++ b/present/doc.go
@@ -13,12 +13,16 @@
 Usage of present:
   -base="": base path for slide template and static resources
   -http="127.0.0.1:3999": host:port to listen on
-  -template="": alternate slide template file
 
 You may use the app.yaml file provided in the root of the go.talks repository
 to deploy present to App Engine:
 	appcfg.py update -A your-app-id -V your-app-version /path/to/go.talks
 
+Input files are named foo.extension, where "extension" defines the format of
+the generated output. The supported formats are:
+	.slide        // HTML5 slide presentation
+	.article      // article format, such as a blog post
+
 The source file format
 
 Source files have the following format.  The first non-blank non-comment