add autobuild status on readme

Author: jtsiomb

README.md

@@ -1,6 +1,10 @@
 spnavcfg
 ========
 
+![GNU/Linux build status](https://github.com/FreeSpacenav/spnavcfg/actions/workflows/build_gnulinux.yml/badge.svg)
+![FreeBSD build status](https://github.com/FreeSpacenav/spnavcfg/actions/workflows/build_freebsd.yml/badge.svg)
+![MacOS X build status](https://github.com/FreeSpacenav/spnavcfg/actions/workflows/build_macosx.yml/badge.svg)
+
 About
 -----
 Spacenav daemon interactive configuration program.