DOC: Update readme

.gitignore

 *.lock
 target/
 .build/
+.cache/
 build/
 build.*/
+.build.*/
 bin/
 pkg/
 tmp/
 fake/
-*.cache/
 *.out
 *.o
 config.log

README.md

@@ -46,6 +46,12 @@ will place all the binaries inside the `target` folder:
 To develop, you can use `cargo run --bin $BINARY` to run the desired binary
 without installing it.
 
+To change the used c++ compiler, you must export the `CXX` variable that
+indicates the correct compiler: `CXX=clang++ cargo build --release`. By default
+the build artifacts for the c++ code are placed in the `.build` folder at the
+root of the project. If you specify another compiler they will be placed in the
+`.build.$(basename "$CXX")` folder.
+
 > **Important note**: Upgrading the database from version 2.4 to version 3 is
 > not possible. You need to delete the database and re-dl all the karas from
 > kurisu.
@@ -102,6 +108,17 @@ For `amadeus`, you can edit its configuration file in the settings section.
 The lektord and related binaries and source code are under the MIT license.
 Please, refer to the [CONTRIBUTING](CONTRIBUTING.md) file for any contributions.
 
+## Building for windows
+
+Windows 64 bit binaries are built from a linux system. On arch, install the
+packages `mingw-w64-gcc`, `mingw-w64-cmake` (aur), `mingw-w64-make` (aur). Make
+sure that you have the windows toolchain for 64 bit installed:
+`rustup target add x86_64-pc-windows-gnu`. Then run the followinf command:
+
+    CXX=x86_64-w64-mingw32-g++ cargo build --target x86_64-pc-windows-gnu
+
+> We need to do something for the Qt6 DLLs and headers...
+
 ---
 
 # TODO List