Document what beta diversity metrics are calculating

[erictleung]

Have a separate tab on more technical details on how each beta diversity metrics are calculated.

[ampatzia]

I've made the following document , mainly copying through phyloseq & Vegan Documentation. All formulas used where formatted to Latex. They can be correctly displayed through mathjax in Shiny . There is an obvious mistake in the w & -1 indices formulas so someone more familiar with these measures should decide which formulas to use through here .