buildman: Convert documentation to rST

Convert the buildman documentation to rST format and include it in the
'build' section.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <foss+uboot@0leil.net>
diff --git a/doc/build/index.rst b/doc/build/index.rst
index 69952f9..9a8105d 100644
--- a/doc/build/index.rst
+++ b/doc/build/index.rst
@@ -11,3 +11,4 @@
    clang
    docker
    tools
+   buildman