README: Update list of supported platforms

Convert to a bullet list.  Drop platforms for which we no longer have
nightly testing.

README.rst

@@ -30,7 +30,13 @@ Building CMake
 Supported Platforms
 -------------------
 
-MS Windows, Mac OS X, Linux, FreeBSD, Solaris, HP-UX, IRIX, BeOS, QNX
+* Microsoft Windows
+* Apple macOS
+* Linux
+* FreeBSD
+* OpenBSD
+* Solaris
+* HP-UX
 
 Other UNIX-like operating systems may work too out of the box, if not
 it should not be a major problem to port CMake to this platform.