Expand readme

Author: simolus3

sqlite3/example/custom_extension/README.md

@@ -1,3 +1,24 @@
 # uuid.c example
 
-Custom SQLite extension (uuid.c) loaded in sqlite3.
+This example shows how to load SQLite extensions into Flutter apps using `sqlite3_flutter_libs`.
+As an example, it uses the [uuid.c](https://github.com/sqlite/sqlite/blob/master/ext/misc/uuid.c)
+extension, but other extensions can be adapted similarly.
+
+To build the extension:
+
+1. A `CMakeLists.txt` (in `src/`) is written for Android, Linux and Windows.
+2. On Android, the NDK is set up to point at the CMake builds in `android/build.gradle`.
+3. For macOS and iOS (which don't use CMake as a build tool), the `uuid.c` file is added
+   to `macos|ios/Classes` instead.
+4. Note that this example does not currently support WebAssembly. Loadable extensions don't
+   work on the web, but you can compile a custom `sqlite3.wasm` bundle with the desired extensions
+   included. The `../custom_wasm_build` example has an example for that setup.
+
+This build setup includes a shared library with the extension in your app. To use it, load the
+`DynamicLibrary` and then load it into sqlite3 like this:
+
+```dart
+  sqlite3.ensureExtensionLoaded(
+    SqliteExtension.inLibrary(uuid.lib, 'sqlite3_uuid_init'),
+  );
+```

sqlite3/example/custom_extension/example/pubspec.yaml

@@ -13,7 +13,7 @@ dependencies:
 
   uuid:
     path: ../
-  
+
   cupertino_icons: ^1.0.6
   sqlite3: ^2.4.6
   sqlite3_flutter_libs: ^0.5.24