Adding documentation to the rebuild script

Since this was reverted (and I/we now know why) I have added
documentation as to avoid the issue re-occuring in the future.

rebuild.js

 var spawn = require('child_process').spawn;
 
-if (process.platform === 'darwin') {
+// Only rebuild the binaries if on Mac OS or Linux, as we're assuming that
+// the toolchain exists. This is here to not rebuild on windows, as that
+// had issues as of https://github.com/andrew/node-sass/issues/123
+if (process.platform === 'darwin' || process.platform === 'linux') {
   spawn('node-gyp', ['rebuild']);
 }