Clarify source build instructions for qt6 branch and cmake compatibility

Add explicit build instructions for the recommended qt6 branch with
the correct git clone command (-b qt6), document the
CMAKE_POLICY_VERSION_MINIMUM workaround for cmake 4.0+ compatibility
with older submodules like eglut, and clearly separate the deprecated
main branch instructions from the recommended qt6 path.

Addresses #120

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: trasnake87

src/source_build/launcher.md

@@ -2,9 +2,9 @@
 
 <div class="warning">
 
-The qt5 version is deprecated and only supported on distributions with protobuf that not depends on abseil and uses cmake 3.x.
+The default (main) branch uses Qt5 and is deprecated. It is only supported on distributions with protobuf that does not depend on abseil and that use cmake 3.x.
 
-Use branch qt6 for cmake 4.0 and newer protobuf.
+**For most users, the `qt6` branch is recommended.** It supports cmake 4.0 and newer protobuf. See the [qt6 branch instructions](#building-the-qt6-branch-recommended) below.
 
 </div>
 
@@ -16,18 +16,33 @@ Use branch qt6 for cmake 4.0 and newer protobuf.
 - **Fedora** (Up to date as of 2024-08-21) - you'll need to install the
   required packages:
   `sudo dnf install clang cmake make git ca-certificates libstdc++ glibc-devel libpng-devel zlib-devel libX11-devel libXi-devel libcurl-devel systemd-devel libevdev-devel mesa-libEGL-devel alsa-lib-devel pulseaudio-libs-devel mesa-dri-drivers systemd-devel libXtst-devel openssl-devel qt5-qtbase-devel qt5-qtwebengine-devel qt5-qtdeclarative-devel qt5-qtsvg-devel qt5-qtquickcontrols qt5-qtquickcontrols2`
-- **Arch** (Up to date as of 2024-12-02) - you'll need to install the 
+- **Arch** (Up to date as of 2024-12-02) - you'll need to install the
   required packages:
   `sudo pacman -S gcc clang ca-certificates openssl libpng libx11 libxi gcc-libs glibc zlib curl systemd libevdev mesa alsa-lib pulseaudio libxtst qt5-base qt5-webengine qt5-declarative qt5-svg qt5-quickcontrols qt5-quickcontrols2`
 - **macOS** - you'll need to install the required packages:
   `brew install cmake libpng openssl@1.1 qt@5`
 
-## Build instructions
+## Building the qt6 branch (recommended)
+
+The `qt6` branch is the actively maintained branch. To build it:
+
+``` bash
+git clone --recursive -b qt6 https://github.com/minecraft-linux/mcpelauncher-manifest.git mcpelauncher && cd mcpelauncher
+mkdir -p build && cd build
+CC=clang CXX=clang++ cmake .. -Wno-dev -DCMAKE_BUILD_TYPE=Release -DJNI_USE_JNIVM=ON
+make -j$(getconf _NPROCESSORS_ONLN)
+```
+
+> **Note:** Some submodules (such as eglut) may specify an older minimum cmake version (e.g. 3.5). If you encounter cmake policy errors when building with cmake 4.0+, try adding `-DCMAKE_POLICY_VERSION_MINIMUM=3.5` to the cmake command.
+
+## Build instructions (main branch, deprecated)
+
+> **Warning:** The main branch uses Qt5 and is deprecated. Prefer the [qt6 branch](#building-the-qt6-branch-recommended) unless you specifically need the Qt5 version.
 
 ``` bash
 git clone --recursive https://github.com/minecraft-linux/mcpelauncher-manifest.git mcpelauncher && cd mcpelauncher
 mkdir -p build && cd build
-CC=clang CXX=clang++ cmake .. -Wno-dev -DCMAKE_BUILD_TYPE=Release -DJNI_USE_JNIVM=ON 
+CC=clang CXX=clang++ cmake .. -Wno-dev -DCMAKE_BUILD_TYPE=Release -DJNI_USE_JNIVM=ON
 make -j$(getconf _NPROCESSORS_ONLN)
 ```