Document macros that are supported in Markdown plugin documentation

Commit 795167c05e60a0c0ba7b5edb00bab85b9d5b104e added support for
macros in the Markdown documentation of a plugin. This feature and the
supported macros were only described in the commit message. Add this
information to the official plugin documentation.

Change-Id: Ie7fcb783015f7667d8ec1c8f869c8f0b5e89e604
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
1 file changed