Lowercase readme.

Former-commit-id: f573efe549e2d48ae551760f866bc8ffda3f24fc

mkdocs.yml

@@ -78,7 +78,7 @@ pages:
       - rbr: file-formats/rbr.md
       - tgf: file-formats/tgf.md
       - xml: file-formats/xml.md
-    - Building the Website: README.md
+    - Building the Website: readme.md
   - FAQ: faq.md
   - About:
     - Release History: RELEASE_HISTORY.md