win32/README.txt: Update SpiderMonkey build instructions

As there are some gotcha's/pitfalls for building SpiderMonkey for use
with GJS on Windows, note in the README file in win32 telling people
how:

-They could obtain the SpiderMonkey sources (which is not easy,
 nowadays)
-They know the flags and envvar's that must be passed into
 SpiderMonkey's configure line for GJS to build and run correctly.
-That they should not try to link to mozglue.lib at any point during the
 GJS build.

Fixes issue #228