Auto merge of rust-lang#45293 - joeratt:patch-1, r=kennytm

Add "Buidling on *nix" sub-header

Since there is a "Build on Windows" section, a simple sub-header here would help Windows users to know they need to keep reading.

Author: bors

README.md

@@ -16,6 +16,7 @@ Read ["Installation"] from [The Book].
 ## Building from Source
 [building-from-source]: #building-from-source
 
+### Building on *nix
 1. Make sure you have installed the dependencies:
 
    * `g++` 4.7 or later or `clang++` 3.x or later