Add README.md for win_toolchain

Add README.md for win_toolchain

I started to build a package for setting up a VS2015 bot, and realized
I didn't really remember how the toolchain setup worked. So I wrote
a readme for this directory.

I see I will live to regret putting way too many "2013"s in various
places in these scripts. :/

R=dpranke@chromium.org
BUG=492774
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=295437

[Dirk Pranke, 2015-05-28 01:21:18]

lgtm w/ one comment.

https://codereview.chromium.org/1150033010/diff/20001/win_toolchain/README.md
File win_toolchain/README.md (right):

https://codereview.chromium.org/1150033010/diff/20001/win_toolchain/README.md...
win_toolchain/README.md:10: the paths, version numbers, etc.
maybe collapse these things into a single bullet, since we're talkinga bout a
single hook invocation here, right? I was initially confused as to whether
Update() was different from vs_toolchain.py update.

[scottmg, 2015-05-28 02:08:50]

https://codereview.chromium.org/1150033010/diff/20001/win_toolchain/README.md
File win_toolchain/README.md (right):

https://codereview.chromium.org/1150033010/diff/20001/win_toolchain/README.md...
win_toolchain/README.md:10: the paths, version numbers, etc.
On 2015/05/28 01:21:18, Dirk Pranke wrote:
> maybe collapse these things into a single bullet, since we're talkinga bout a
> single hook invocation here, right? I was initially confused as to whether
> Update() was different from vs_toolchain.py update.

Done.

[scottmg, 2015-05-28 02:10:18]

The CQ bit was checked by scottmg@chromium.org

[scottmg, 2015-05-28 02:10:19]

The patchset sent to the CQ was uploaded after l-g-t-m from dpranke@chromium.org
Link to the patchset: https://codereview.chromium.org/1150033010/#ps40001
(title: "merge bullets")

[commit-bot: I haz the power, 2015-05-28 02:10:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1150033010/40001

[commit-bot: I haz the power, 2015-05-28 02:12:12]

Committed patchset #3 (id:40001) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=295437