Markdown
Crawler.ID2MD is a Crawler-based software product which provides conversion from InDesign to Markdown syntax.
The Markdown syntax definitions can be found here:
http://daringfireball.net/projects/markdown/syntax
The product is available as a .zip archive. To install, you need to first decompress the .zip archive and get to a folder 'Crawler.ID2MD' which contains a file called Export.jsxbin.
Export.jsxbin is the script you'll need to run to activate Crawler.ID2MD.
Installation
To install Crawler.ID2MD, you first need to launch InDesign, and find the Scripts panel. If it is not visible, you can make it appear by means of the Window - Utilities - Scripts menu item.
Once the Scripts panel is visible, right-click or <Control>-click the User entry and select 'Reveal in Finder' (on Mac) or 'Reveal in Explorer' (on Windows).
A window on a folder called Scripts should open.
Inside there should be a folder called Scripts Panel. Double-click its icon to enter it.
Once you're inside the Scripts Panel folder, you can drag the Crawler.ID2MD folder into it.
Now switch back to InDesign, and verify that the Crawler.ID2MD folder has appeared under the User folder on the Scripts panel:
Click the disclosure triangle - you should now see Export.jsxbin.
Running
Open an InDesign document, and double-click Export.jsxbin on the Scripts panel.
A new file with the same name as the original document should appear. The file name extension of the new file is .md by default. If your InDesign document is called MyDocument.indd you should see MyDocument.md appear next to it.
If the InDesign document has graphics in it, these will be exported into a separate folder called images by default.