Repeat warning comment about keeping "What's included" sections in sync

BS3 commits: 7e758f60fcb8f5391ea48cacf432c76e8b747909
Update source maps info in README.md.

BS3 commits: 1dd665c3ab7baed8e6a76245b022e02d14a04dc1
Catch and properly report compile errors that Less.js's toCSS() can throw

BS3 commits: f1102430eca4b498294b457020bedda8a8890d52