Show Travis and AppVeyor build results of the master branch in the
readme (and project page). Use images from shields.io so shields
look the same and a label can be added.
Closes#140
Now that we have CONTRIBUTING.md there doesn't seem to be any reason for not
having README.md as well.
Perhaps this one will replace docs/readme.txt in the future.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76692 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775