also mention libattr

ACLs are implemented as xattrs, so libattr is pulled in as a dependency of libacl.

docs/global.rst.inc

@@ -13,6 +13,7 @@
 .. _PBKDF2: https://en.wikipedia.org/wiki/PBKDF2
 .. _ACL: https://en.wikipedia.org/wiki/Access_control_list
 .. _libacl: http://savannah.nongnu.org/projects/acl/
+.. _libattr: http://savannah.nongnu.org/projects/attr/
 .. _liblz4: https://github.com/Cyan4973/lz4
 .. _OpenSSL: https://www.openssl.org/
 .. _Python: http://www.python.org/

docs/installation.rst

@@ -8,7 +8,7 @@ Installation
 
 * Python_ >= 3.2
 * OpenSSL_ >= 1.0.0
-* libacl_
+* libacl_ (that pulls in libattr_ also)
 * liblz4_
 * some python dependencies, see install_requires in setup.py