Add README.md and CONTRIBUTING.md, refs #17189
Add back submodule for less v1.3.3.

This is needed to get dijit/themes/claro/compile.js and dojox/trunk/mobile/themes/utils/compile.sh
working again.  Might be possible to install less via "npm install" instead but this is the path of
least resistance.  Less submodule was originally part of 8525a1a318033ea75bef49c54c1e91688aad49e9.