Add a link to Mojo documentation main page to the document index

Change-Id: I7c6d15d75fd6ea4a936729bc2b07445778fae165
Reviewed-on: https://chromium-review.googlesource.com/c/1281867Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600057}

docs/README.md

@@ -330,6 +330,7 @@ used when committed.
     install Chromium OS on VMWare.
 *   [User Data Directory](user_data_dir.md) - How the user data and cache
     directories are determined on all platforms.
+*   [Mojo](../mojo/README.md) - IPC mechanism used by services.
 
 ### Probably Obsolete
 *   [TPM Quick Reference](tpm_quick_ref.md) - Trusted Platform Module notes.