Update the README after recent changes.

Update the platform list after dropping Win9x and OS/2 support.

Use new URLs after web site redesign and Trac upgrade.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76691 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Added a brief version of README in markdown for Github users.

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