More docs

Author: josevalim

lib/mix/lib/mix.ex

@@ -359,7 +359,11 @@ defmodule Mix do
     * `MIX_OS_CONCURRENCY_LOCK` - when set to `0` or `false`, disables mix compilation locking.
       While not recommended, this may be necessary in cases where hard links or TCP sockets are
       not available. When opting for this behaviour, make sure to not start concurrent compilations
-      of the same project.
+      of the same project
+
+    * `MIX_OS_DEPS_COMPILE_PARTITION_COUNT` - when set to a number greater than 1, it enables
+      compilation of dependencies over multiple operating system processes. See `mix help deps.compile`
+      for more information
 
     * `MIX_PATH` - appends extra code paths