Documentation: add a suggested start to the index

Now that the documentation is organised into categories, it's less
clear from the sidebar where somebody should go to get started.  So
let's add a note encouraging them to try the "Getting Spectrum"
tutorial first.

1 files changed, 3 insertions, 0 deletions

diff --git a/Documentation/index.adoc b/Documentation/index.adoc
index e1d6d63..6bfa2e6 100644
--- a/Documentation/index.adoc
+++ b/Documentation/index.adoc
@@ -3,3 +3,6 @@
 
 // SPDX-FileCopyrightText: 2022 Alyssa Ross <hi@alyssa.is>
 // SPDX-License-Identifier: GFDL-1.3-no-invariants-or-later OR CC-BY-SA-4.0
+
+If you're new to Spectrum, you probably want to start with
+xref:getting-spectrum.adoc[Getting Spectrum].