Make sure README.md mentions that make can build in parallel.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

README.md

@@ -62,9 +62,14 @@ dblatex.
 
     $ perl boot
     $ ./configure
-    $ make
+    $ make         # can also say 'make -jX' for X number of jobs
     $ make install
 
+(NB: **Do you have multiple cores? Be sure to tell that to `make`!** This can
+save you hours of build time depending on your system configuration, and is
+almost always a win regardless of how many cores you have. As a simple rule,
+you should have about N+1 jobs, where `N` is the amount of cores you have.)
+
 The `perl boot` step is only necessary if this is a tree checked out
 from git.  For source distributions downloaded from [GHC's web site] [1],
 this step has already been performed.