README: add building instructions

README.md

@@ -6,6 +6,15 @@ HOS requires:
 * A UEFI system.
 * 128MB of RAM.
 
+# Building HOS
+
+From the top level of the repository, run:
+
+    ./rscons
+
+If you get errors about missing packages, install the packages and then re-run
+the build command.
+
 # Running HOS
 
 ## Physical Machine