[libglnx] README.md: Add some porting bits

[Colin Walters <walters src gnome org>]

commit d8a3d3b821dc441475638d2d38a62a57f83d3d28
Author: Colin Walters <walters verbum org>
Date:   Fri Mar 20 11:53:57 2015 -0400

    README.md: Add some porting bits

 README.md |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)
---
diff --git a/README.md b/README.md
index 1767a2d..5627c6a 100644
--- a/README.md
+++ b/README.md
@@ -2,3 +2,17 @@ libglnx is the successor to libgsystem: https://git.gnome.org/browse/libgsystem
 
 It is for modules which depend on both GLib and Linux, intended to be
 used as a git submodule.
+
+Porting from libgsystem
+-----------------------
+
+For all of the filesystem access code, libglnx exposes only
+fd-relative API, not `GFile*`.  It does use `GCancellable` where
+applicable.
+
+For local allocation macros, you should start using the `g_auto`
+macros from GLib.  A backport is included in libglnx.  There are a few
+APIs not defined in GLib yet, such as `glnx_fd_close`.
+
+`gs_transfer_out_value` is replaced by `g_steal_pointer`.
+