Describe how to use the custom saving functions in the README.

Author: LTLA

README.md

@@ -130,6 +130,48 @@ an_index.save(path_prefix);
 auto reloaded = knncolle::load_prebuilt(path_prefix);
 ```
 
+For custom Annoy-related types and distances, users can define their own saving and loading methods.
+For example, if we had a custom distance:
+
+```cpp
+// Mock up a custom distance.
+class MyCustomDistance : public Annoy::Euclidean {};
+
+// Define a function to save information about this custom distance during .save() calls.
+knncolle_annoy::custom_save_for_annoy_data<MyCustomDistance>() = [](const std::string& prefix) -> void {
+    const auto path = prefix + "_custom";
+    // write something about MyCustomDistance at 'path'.
+};
+
+// Make a new loading function that uses the saved custom information. 
+reg[knncolle_annoy::save_name] = [](const std::string& prefix) -> Prebuilt<int, double, double>* {
+    auto config = knncolle_annoy::scan_prebuilt_save_config(prefix);
+
+    if (config.index != knncolle_annoy::get_numeric_type<int>()) {
+        throw std::runtime_error("unexpected type for the Annoy index");
+    }
+    if (config.data != knncolle_annoy::get_numeric_type<float>()) {
+        throw std::runtime_error("unexpected type for the Annoy data");
+    }
+
+    if (config.distance == "euclidean") {
+        return knncolle_annoy::load_annoy_prebuilt<int, double, double, Annoy::Euclidean>();
+    } else if (config.distance == "manhattan") {
+        return knncolle_annoy::load_annoy_prebuilt<int, double, double, Annoy::Manhattan>();
+    } else {
+        const auto info = prefix + "_custom";
+        // read something about the custom distance.
+        return knncolle_annoy::load_annoy_prebuilt<int, double, double, MyCustomDistance>();
+    }
+};
+
+std::string custom_prefix = "anno/location/custom_";
+knncolle_annoy::AnnoyBuilder<int, double, double, MyCustomDistance> custom_builder;
+auto custom_index = custom_builder.build_unique(mat);
+custom_index.save(custom_prefix);
+auto custom_reloaded = knncolle::load_prebuilt(custom_prefix);
+```
+
 ## Building projects 
 
 ### CMake with `FetchContent`