update readme; fix priority bit size

Author: myk002

LibWrapper.cpp

@@ -40,7 +40,7 @@ LibWrapper::LibWrapper(const string& fname) {
     if (!handle)
         return;
 
-    int * priority_sym = (int *)load_sym(handle, "dfhooks_priority");
+    int32_t * priority_sym = (int *)load_sym(handle, "dfhooks_priority");
     if (priority_sym) priority = *priority_sym;
 
     init = (dfhooks_init_fn)load_sym(handle, "dfhooks_init");

LibWrapper.h

@@ -13,7 +13,7 @@ typedef bool (*dfhooks_ncurses_key_fn)(int key);
 
 struct LibWrapper {
     void* handle = nullptr;
-    int priority = 0;
+    int32_t priority = 0;
     dfhooks_init_fn init = nullptr;
     dfhooks_shutdown_fn shutdown = nullptr;
     dfhooks_update_fn update = nullptr;

README.md

@@ -1,2 +1,24 @@
 # dfhooks
-Dwarf Fortress dfhooks API chainloader
+Dwarf Fortress dfhooks API chainloader.
+
+This library implements the dfhooks API as (defacto) defined in the DF
+[g\_src](https://github.com/Putnam3145/Dwarf-Fortress--libgraphics--/blob/master/g_src/dfhooks.cpp)
+sources and propagates the calls to other libraries that similarly implement
+the API.
+
+Those libraries must also be installed in the top-level Dwarf Fortress game
+directory and conform to the following per-platform naming scheme:
+
+- Windows: `dfhooks_{name}.dll` (e.g. `dfhooks_mylib.dll`)
+- Linux: `libdfhooks_{name}.so`
+- MacOS: `libdfhooks_{name}.dylib`
+
+In addition to the symbols required by the dfhooks API, a library can also
+export a `dfhooks_priority` symbol, which must be of type `int32_t`. Libraries
+with lower values in the exported `dfhooks_priority` will be called first when
+callbacks are propagated, except for shutdown, for which libraries with lower
+values will be called last. If a library does not export a priority, the value
+0 is used.
+
+If you need to position yourself relative to DFHack, DFHack exports a priority
+of 100.