docs: Add bootc

Let's document the state of this.

docs/bootc.md

@@ -0,0 +1,28 @@
+---
+parent: Experimental features
+nav_order: 1
+---
+
+# bootc
+
+rpm-ostree vendors the [containers/bootc project](https://github.com/containers/bootc) as
+an experimental feature; this avoids needing a separate package.
+
+## Enabling bootc
+
+As part of a container build:
+
+```
+$ ln -sr /usr/bin/rpm-ostree /usr/bin/bootc
+```
+
+In the future we are likely to pursue deeper integration as bootc gains more features.
+
+## Compatibility
+
+It's very much an intentional design feature that `bootc` is compatible with
+a container-deployed rpm-ostree system.  You can seamlessly invoke e.g.
+`bootc upgrade` as well as `rpm-ostree upgrade` for example.
+
+However, as soon as any "machine local" changes are set up, such as layered
+packages or local initramfs regeneration, `bootc upgrade` will no longer work.

docs/experimental.md

@@ -7,6 +7,7 @@ nav_order: 7
 # Experimental features
 {: .no_toc }
 
+1. [bootc](bootc.md)
 1. [Wrapping other CLI entrypoints](cliwrap.md)
 1. [ostree native containers](container.md)
 1. [rebuild](ex-rebuild.md)