add a note about llfuse requirements

setup.py

@@ -41,6 +41,11 @@ install_requires = []
 # if you do not have llfuse, do not require it, most of borgbackup will work.
 extras_require = {
     'fuse': [
+        # note: the C code generated by cython and bundled into the pypi package
+        # is the reason for these requirements. Pre-1.3.7 it was made by a too old
+        # cython to work on py39. In 1.3.7 it was made by a cython version that
+        # produces py39 compatible code, but that also led to a freebsd compile issue.
+        # if you cythonize yourself, you can use either version.
         'llfuse >=1.3.4, <1.3.7; python_version <"3.9"',  # broken on py39
         'llfuse >=1.3.7, <2.0; python_version >="3.9"',  # broken on freebsd
     ],