optional "endTime" added to chapter spec

chapters/jsonChapters.md

@@ -39,7 +39,7 @@ The "chapter" object takes this basic form:
 
 There is only one required attribute:
 
- - `startTime` (required - float) The time, expressed in seconds with float precision for fractions of a second.
+ - `startTime` (required - float) The starting time of the chapter, expressed in seconds with float precision for fractions of a second.
 
 #### Optional Attributes:
 
@@ -47,6 +47,7 @@ There is only one required attribute:
  - `img` (optional - string) The url of an image to use as chapter art.
  - `url` (optional - string) The url of a web page or supporting document that's related to the topic of this chapter.
  - `toc` (optional - boolean) If this property is present and set to false, this chapter should not display visibly to the user in either the table of contents or as a jump-to point in the user interface.  It should be considered a "silent" chapter marker for the purpose of meta-data only.  If this property is set to `true` or not present at all, this should be considered a normal chapter for display to the user.  The name "toc" is short for "table of contents".
+ - `endTime` (optional - float) The end time of the chapter, expressed in seconds with float precision for fractions of a second.
 
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