MINOR: 51d: Added dummy libraries for the 51Degrees module for testing.

These are intended for use by HAProxy developers to ensure any changes
did not affect the 51Degrees implementation. The 51Degrees module can be
enabled and used by using the source in contrib/51d. This will run
without breaking, but will not return any meaningful information.

This is ideal for testing HAProxy core code, and other modules alongside
51Degrees, but should never be used as an actual module as it does
nothing.
diff --git a/doc/51Degrees-device-detection.txt b/doc/51Degrees-device-detection.txt
index 381008a..2d4679d 100644
--- a/doc/51Degrees-device-detection.txt
+++ b/doc/51Degrees-device-detection.txt
@@ -53,6 +53,15 @@
 Trie data. Free Hash Trie data file can be obtained by signing up for a licence
 key at https://51degrees.com/products/store/on-premise-device-detection.
 
+For HAProxy developers who need to verify that their changes didn't affect the
+51Degrees implementation, a dummy library if provided in the contrib/51d
+directory. This does not function, but implements the API such that the
+51Degrees module can be used (but not return any meaningful information). To
+test either Pattern or Hash Trie, build with:
+    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/pattern
+or
+    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/trie
+respectively.
 
 The configuration file needs to set the following parameters: