move 3rd party docs/license file from package dir to docs/3rd_party

setup.py excludes all .c .h .pyx files from installation, but such
docs / license files would still be copied to the target directory
if they are inside the python "borg" package dir.

(cherry picked from commit 831a06a07d93626b33b094f75c63ae017c09974f)

docs/3rd_party/README

@@ -0,0 +1,5 @@
+Here we store 3rd party documentation, licenses, etc.
+
+Please note that all files inside the "borg" package directory (except the
+stuff excluded in setup.py) will be INSTALLED, so don't keep docs or licenses
+there.