doc: ulib: Provide some documentation on the ulib examples

Update the documentation to point to the examples.

Series-to: concept
Cover-letter:
ulib: Provide examples for building outside the tree
The ulib tests are built within the U-Boot build system, so make use of
various settings which are provided.

When using ulib outside the U-Boot tree (and outside its build
environment), things are a little more tricky.

This series provides a few simple examples to illustrate how to make
it work. An example Makefile is provided as well.

The same Makefile is used from CI just to ensure that it continues to
work as expected.
END

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/ulib.rst

@@ -88,6 +88,9 @@ u-boot-lib.h you will have problems, as described in the following sections.
 
 This will be addressed with future work.
 
+With that caveat, see example/ulib/README for instructions on how to use the
+provided example.
+
 Including U-Boot header files from outside
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~