Add usage instructions.

Author: bottiger1

readme.txt

@@ -1,3 +1,10 @@
+How to use
+
+1. Download the binary by clicking Releases on the right on github.
+2. Put it in sourcemod/extensions.
+3. Copy sourcepawn/msgpack.inc to your sourcemod/scripting/includes folder.
+4. You can see an example of how to include it in sourcepawn/msgpack-tests.sp
+
 How to compile
 
 1. Install ambuild.