Improve README.bootstrap

During the bootstrap process it is necessary to clean the build tree between
the initial and complete builds.  This patch describes this and adds
commands to the instructions that clean the build tree.

As compilation can take a long time I've also added a paragraph describing
how to instruct make to run multiple parallel tasks.

README.bootstrap:
    As above.

README.bootstrap

@@ -41,10 +41,13 @@ further need for tarballs when building Mercury from Git.
 
 BOOTSTRAPPING
 -------------
-Once the minimal prerequisite compiler has been built, a reasonably complete
-distribution can be bootstrapped by issuing the following command sequence:
+Once the minimal prerequisite compiler has been built and installed, a
+reasonably complete distribution can be bootstrapped.  However the build
+tree must be clean or the build may fail.  Clean and then build the complete
+distribution by issuing the following command sequence:
 
 ----
+make realclean
 aclocal -I m4
 autoconf
 ./configure
@@ -66,6 +69,17 @@ components:
 People wanting to simply experiment with Mercury would be best-served with
 using only the `asm_fast.gc` grade or `hlc.gc` on OS X.
 
+Since building Mercury can take a long time, you may prefer to use multiple
+processors during the build process.  Adding "PARALLEL=-jN" without the
+quotes to make's command line will tell make to run N tasks at once.
+Substitute N with the number of processors in your system, or fewer if you
+want to use other processors for other tasks.  For example:
+
+----
+make PARALLEL=-j4
+sudo make PARALLEL=-j4 install
+----
+
 ALTERNATIVES TO BOOTSTRAPPING
 -----------------------------
 Unless you are planning to contribute to the Mercury project, bootstrapping