doc: Add documentation for how to build U-Boot host tools

This adds a reST document for how to build U-Boot host tools,
including information for both Linux and Windows.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/index.rst b/doc/index.rst
index 458f0d2..206a045 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -15,6 +15,17 @@
 .. toctree::
    :maxdepth: 2
 
+User-oriented documentation
+---------------------------
+
+The following manuals are written for *users* of the U-Boot - those who are
+trying to get it to work optimally on a given system.
+
+.. toctree::
+   :maxdepth: 2
+
+   build/index
+
 Unified Extensible Firmware (UEFI)
 ----------------------------------