cffi-posix/fd-gray

diff --git a/README.md b/README.md
index 1221d61..d825131 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,10 @@
 # fd-gray
 
+Unix file descriptor gray streams are binary gray streams operating
+on `(unsigned-byte 8)` data using `unistd:read`, `unistd:write` and
+`unistd:close` to embed Unix file descriptors in standard Common Lisp
+streams.
+
 ### fd-gray:input-stream *fd* --> *stream*
 
 Creates an input stream for file descriptor *fd*.