add contribute cards to index

index.md

@@ -20,3 +20,7 @@ The How-to cards are intended to provide practical guidance in implementing Data
   * [Registering human-subject data to DAISY](./external/daisy/)
   * [VPN-Cerbere](./external/vpn-cerbere-access/)
   * [Installing-Owncloud-Client](./external/installing-owncloud-client/)
+  * git, markdown, and how to contribute to a repository
+    * [Git clients](./external/contribute/git-clients/)
+    * [Markdown](./external/contribute/write-markdown/)
+    * [How to add or edit a howto card](./external/contribute/add-edit-card/)
\ No newline at end of file