updated README to show CI build status

Author: jtsiomb

README.md

@@ -1,6 +1,9 @@
 meshfile library
 ================
 
+![GNU/Linux build status](https://github.com/jtsiomb/meshfile/actions/workflows/build_gnulinux.yml/badge.svg)
+![Windows (MinGW) build status](https://github.com/jtsiomb/meshfile/actions/workflows/build_win_mingw.yml/badge.svg)
+
 About
 -----
 Meshfile is a simple C library for reading and writing 3D mesh file formats. It