Added link to GitHub & Critic PR handling 101

TSIA.
auto merge of #1595 : brunoabinader/servo/patch-1, r=larsbergstrom

TSIA.
auto merge of #1593 : pcwalton/servo/dom-leaf-set, r=metajack

construction.

This will be very slow until we have the concurrent hash table, but we
might as well get it in.

r? @metajack
layout: Make box building thread safe by pushing down styles into
non-element children during the cascade.

Fetching them from the parent isn't thread-safe.

Adds a test for anonymous block box inheritance.