Recommend parallel builds

Load average of 0.9 is something that even a PhD student's crappy
laptop computer should be able to handle, without completely killing
it.

It really should be something like:

 make -j -l $(echo \($(grep "^processor" /proc/cpuinfo)+1\)\*0.9 | bc)

but that is probably far too complex to put in a README. The average
developer should be able to pick a reasonable value themselves.

README

@@ -97,7 +97,7 @@ There are two ways to build the opm-core library:
  cd opm-core
  autoreconf -i
  ./configure
- make
+ make -j -l 0.9
 If you want to install the library:
  make install
 or (if installing to /usr/local or similar)