Add build status links to gyp.gsrc.io

Windows is on AppVeyor, Linux & Mac on Travis.

Those systems are only building gyptest.py -f ninja at the moment.
The other configurations don't have any specific problem, other than
the gyp code appears to need some bug fixes.

The Windows build is also set to use a VS2013 image, rather than
2015 or 2017, again because of assumptions in the tests that would
need to be addressed.

Change-Id: Ifb204d44068622027ef8bd6bc95e58f0f9eb48d9
Reviewed-on: https://chromium-review.googlesource.com/900027


Reviewed-by: Mark Mentovai <mark@chromium.org>

index.md

@@ -6,6 +6,10 @@ Source code: https://chromium.googlesource.com/external/gyp
 
 Mailing list:  http://groups.google.com/group/gyp-developer
 
+Build status:
+- Windows: [![Build status](https://ci.appveyor.com/api/projects/status/60c6eoat3e5lojly/branch/master?svg=true)](https://ci.appveyor.com/project/scottg/gyp/branch/master)
+- Linux and Mac: [![Build status](https://travis-ci.org/chromium/gyp.svg?branch=master)](https://travis-ci.org/chromium/gyp)
+
 GYP is intended to support large projects that need to be built on multiple
 platforms (e.g., Mac, Windows, Linux), and where it is important that
 the project can be built using the IDEs that are popular on each platform