Squashed 'site_scons/' changes from 80fb464..03c1d29

03c1d29 Update README to make it clear you need to use the whole repo as site_scons

git-subtree-dir: site_scons
git-subtree-split: 03c1d294d46d9ad44790d1bee100fe3d4d8f3f00
Merge commit '83440b89048cd07c64abc06d94eec41aab83a890'