libbtrfsutil: python: reuse existing README.md for long description

Instead of copying the file during custom build commands, just use a
soft link to re-use the existing README.d from libbtrfsutil.

Issue: #310
Signed-off-by: Qu Wenruo <wqu@suse.com>

libbtrfsutil/python/README.md

@@ -0,0 +1 @@
+../README.md

libbtrfsutil/python/setup.py

@@ -108,6 +108,8 @@ setup(
     #version=get_version(),
     version='6.12',
     description='Library for managing Btrfs filesystems',
+    long_description=open('README.md').read(),
+    long_description_content_type='text/markdown',
     url='https://github.com/kdave/btrfs-progs',
     license='LGPLv2+',
     cmdclass={'build_ext': my_build_ext},