Update README to describe bundleSFX arithmetic
Update README to describe bundleSFX arithmetic

Added documentation to warn users that SFX bundles can only use addition and wildcard arithmetic.
lookfirst/builder,richardTowers/builder,matthewbauer/builder,ahmedshuhel/builder,richardTowers/builder,alexisvincent/builder,matthewbauer/builder,synaptek/builder,jrauschenbusch/builder,alexisvincent/builder,ahmedshuhel/builder,whitneyit/builder,tonytanger/builder,JeroenVinke/builder,systemjs/builder,Psvensso/builder,buddhike/builder,douglasduteil/builder,robwormald/builder,tonytanger/builder,jrauschenbusch/builder,lookfirst/builder,douglasduteil/builder,alexandr2110pro/builder,Psvensso/builder,synaptek/builder,Munter/builder,alexandr2110pro/builder,jrauschenbusch/builder,systemjs/builder,JeroenVinke/builder,whitneyit/builder,Munter/builder,systemjs/builder,alexisvincent/builder,robwormald/builder,buddhike/builder