Add prebuilt bindings for GDAL 3.6 and document how to do it

Fixes georust#351

gdal-sys/Readme.md

@@ -37,3 +37,12 @@ attempt to use prebuilt bindings corresponding to that version. Alternatively,
 you can generate your own bindings from your libgdal installation by specifying
 the `bindgen` feature.
 
+## Creating prebuilt bindings
+
+If a new version of GDAL is released, you (as a `gdal` contributor) can
+generate new bindings for inclusion in the `gdal-sys` source distribution by
+building with the `bindgen` feature, and then copying the generated file. For
+example (the hash will differ in your build):
+
+    $ cargo build --features bindgen
+    $ cp target/debug/build/gdal-sys-db833e3088b78e57/out/bindings.rs gdal-sys/prebuilt-bindings/gdal_3.6.rs

gdal-sys/build.rs

@@ -22,8 +22,9 @@ pub fn write_bindings(include_paths: Vec<String>, out_path: &Path) {
         .allowlist_function("VSI.*");
 
     for path in include_paths {
-        builder = builder.clang_arg("-I");
-        builder = builder.clang_arg(path);
+        builder = builder
+            .clang_arg("-I").clang_arg(path)
+            .clang_arg("-fretain-comments-from-system-headers");
     }
 
     builder