binman: doc: Add documentation to htmldocs

Add a link to binman's documentation and adjust the files so that it is
accessible. Use the name README.rst so it is easy to discover when binman
is installed without U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/fit.rst b/doc/usage/fit.rst
new file mode 100644
index 0000000..7037434
--- /dev/null
+++ b/doc/usage/fit.rst
@@ -0,0 +1,8 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Flat Image Tree (FIT)
+=====================
+
+U-Boot uses Flat Image Tree (FIT) as a standard file format for packaging
+images that it it reads and boots. Documentation about FIT is available at
+doc/uImage.FIT