Add tips to optimizing Jekyll build time

[sawyerh]

I don't have a comprehensive list of tips, but maybe we can start compiling one:

• Exclude node_modules, bower_components, etc: http://stackoverflow.com/questions/26855552/jekyll-compiling-seems-way-too-slow?answertab=votes#tab-top

[larryfox]

• Limit large for loops
• Limit nested for loops