docs/stdlib: clarify 0.4 vs main branch

Signed-off-by: Stephen Gutekanst <[email protected]>

content/docs/stdlib.md

@@ -13,9 +13,33 @@ The Mach standard library can be found at: [github.com/hexops/mach](https://gith
 
 Thanks to Zig's lazy code evaluation and lazy dependency fetching, you really only pay for the parts you use.
 
-## Getting started
+## Choosing your Mach version
 
-Add the Mach dependency to your `build.zig.zon` file:
+### 0.4 branch (not actively developed, but more stable)
+
+The 0.4 branch is what applications like [Pixi](https://github.com/foxnne/pixi) and games like `#lordofzero` (in the Discord) are using. It uses GLFW, WebGPU, and optionally the ECS. It is [no longer in development](https://discord.com/channels/996677443681267802/996688886216523907/1259736837648416829) however.
+
+To use this version, add the Mach dependency to your `build.zig.zon` file:
+
+<pre><code id="zig-fetch">zig fetch --save https://pkg.machengine.org/mach/$LATEST_COMMIT.tar.gz
+</code></pre>
+<script>
+fetch('https://api.github.com/repos/hexops/mach/branches/0.4', {
+  method: 'GET',
+  headers: {'Accept': 'application/json'}},
+)
+  .then(resp => resp.json())
+  .then(resp => {
+    let elem = document.querySelector('#zig-fetch');
+    elem.innerHTML = elem.innerHTML.replace('$LATEST_COMMIT', resp.commit.sha);
+  });
+</script>
+
+### main branch (actively developed, experimental/unstable)
+
+This branch is where active development is happening, we are removing GLFW and WebGPU in favor of our own packages ([more details](https://discord.com/channels/996677443681267802/996688886216523907/1259736837648416829)) which means it is not stable - or may not even run - on some OS. At the time of writing Sep 30th, Windows is relatively stable, Linux might work under Wayland, and macOS is not expected to work currently.
+
+To use this version, add the Mach dependency to your `build.zig.zon` file:
 
 <pre><code id="zig-fetch">zig fetch --save https://pkg.machengine.org/mach/$LATEST_COMMIT.tar.gz
 </code></pre>
@@ -31,6 +55,8 @@ fetch('https://api.github.com/repos/hexops/mach/branches/main', {
   });
 </script>
 
+## Using the dependency
+
 Then make it importable by your code in `build.zig`, e.g.:
 
 ```zig