README: Add explicit link to Markdown documentation

Change-Id: I747a5752881b253c3304b8ae2071e0cf3966f500

README.md

diff --git a/README.md b/README.md
index 695d3a1..3bbfb98 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,10 @@
 guiding principle is simplicity: it has no formal access controls, no write
 access, no fancy Javascript, etc.
 
+Gitiles automatically renders `*.md` Markdown files into HTML for simplified
+documentation. Refer to the [Markdown documentation](/Documentation/markdown.md)
+for details.
+
 ## Contributing to Gitiles
 
 Please refer to the [Developer Guide](/Documentation/developer-guide.md).