Rework README to avoid broken "python script/scons.py" calls.

Rework README to avoid broken "python script/scons.py" calls.

While it is nice to mention the whole SCONS_LIB_DIR bit, it seems
easier and less error prone to just stick to bootstrap.py.  Maybe not
exactly what bootstrap.py was intended for, but certainly something it
is very good at.  The new README layout (hopefully) makes this
more obvious.

I also added "-o test.log" to to the DEVELOPMENT WORKFLOW runtest.py
invocation, since it would be a shame to run that many tests, and then
have to run them all again if the failures scrolled up out of the
terminal and you weren't running xterm/screen/etc. with a history.