sandbox: Add documentation about required/useful packages

Quite a few packages are used by sandbox or tools. Add a list of these to
help people setting up for the first time.
Signed-off-by: NSimon Glass <sjg@chromium.org>

doc/arch/sandbox.rst

@@ -34,6 +34,16 @@ integers can only be built on 64-bit hosts.
 Note that standalone/API support is not available at present.
 
 
+Prerequisites
+-------------
+
+Here are some packages that are worth installing if you are doing sandbox or
+tools development in U-Boot:
+
+   python3-pytest lzma lzma-alone lz4 python3 python3-virtualenv
+   libssl1.0-dev
+
+
 Basic Operation
 ---------------