List compatible tools too

Specifically, khard inter-operates with pimsync and vdirsyncer quite
well and is worth mentioning.

Author: WhyNotHugo

content/_index.md

@@ -8,10 +8,18 @@ pimutils is a suite of programs for managing contacts, calendars and tasks on th
 * [pimsync](https://pimsync.whynothugo.nl/): re-implementation of vdirsyncer
   addressing outstanding issue and limitations.
 
-These programs are build around existing standards formats and protocols, and
-use the [`vdir` storage format][vdir] to store data on a local filesystem. The
-`vdir` specification builds atop existing standards, and allows applications
+These programs are build around existing standards formats and protocols. For
+storing data locally, they use the [`vdir` storage format][vdir]. The `vdir`
+specification itself builds atop existing standards, and allows applications
 from different authors to operate with the same set of locally stored calendars
 and address book.
 
 [vdir]: /specs/vdir/
+
+The following tools also work on [vdir] storages:
+
+* [khard](https://github.com/lucc/khard): Command line vCard client
+* [ab-bday](https://git.sr.ht/~whynothugo/ab-bday): Generate birthday iCalendar
+  events for vCard contacts.
+* [ab-tidy](https://git.sr.ht/~whynothugo/ab-tidy): Bulk-rename vCard files to
+  match the formatted name of the contact.