I was building the cpython code a while back, and noticed that the build system was using msbuild, when 
PCBuild\built.bad -e -d 
was ran. Upon further investigation, it seems that the /m compiler flag is not included. This flag would allow for concurrent builds, considerably speeding up the build time. Therefore, I would like to propose the addition of the /m flag to the build.bat file, so that builds on multi-core systems can be considerable accelerated.

No need, you can supply it yourself :).  Try 'PCbuild\build.bat -d -e /m'.  That won't work on 3.4, but will on 3.5+; I specifically made sure extra arguments were passed along to msbuild when I rewrote PCbuild\build.bat a while back just so that this would work.

(I don't want to make /m the default and force those who don't want it to turn it off because I've had some odd results using it before that I haven't had a chance to look into.)

On the other hand, I did fail to document that little gem.  It should be added to the section about build.bat in PCbuild/readme.txt.

Would you mind letting me do the honors? First time contributing and all that :P

Of course, I'll be happy to review and commit for you.

I uploaded the file, just checking if it hasn't come through.

Patch came through fine; you've done perfectly process-wise.  I'll
post a review of the patch soon.

Ok, thank you. Just wanted to confirm this, as I plan to help out on the code/documentation as well (and hence want to understand the process beforehand). I look forward to working on Python in the future.

I finally managed to get a review posted on Rietveld, which should have sent you an email (sorry for the delay!).

How do you want me to about fixing the issues you suggested - another patch?

Another patch would be perfect :)

Uploaded a new patch.

New changeset f35403bf8c91 by Zachary Ware in branch 'default':
Issue #22261: Add a note to PCbuild\readme.txt about MSBuild switches.
https://hg.python.org/cpython/rev/f35403bf8c91

I finally made it back to this and committed a tweaked version of your patch.  Thanks for the report and patch!