vdir: link to spec from home page

Author: WhyNotHugo

content/_index.md

@@ -4,3 +4,11 @@ pimutils is a suite of programs for managing contacts, calendars and tasks on th
 * [todoman](https://github.com/pimutils/todoman) is a command line interface tasks manager
 * [vdirsyncer](https://github.com/pimutils/vdirsyncer) can synchronize calendar data, including data from khal and todoman
 * [pimsync](https://pimsync.whynothugo.nl/) is a re-implementation of vdirsyncer addressing outstanding issue and limitations.
+
+These tools are build around existing standards formats and protocols.
+The `vdir` storage format also builds atop existing standards:
+
+* [vdir] is a convention for storing calendars and address books in a
+  filesystem.
+
+[vdir]: http://localhost:1313/specs/vdir/