README.md: add a "Building" section

Document how to build the base images locally. Link to relevant
documentation pages.

README.md

@@ -9,6 +9,25 @@ been extremely successful. This project aims to apply the same technique for
 bootable host systems - using standard OCI/Docker containers as a transport and
 delivery format for base operating system updates.
 
+## Building
+
+First, the expectation is that most users will want to build *layered* images
+on top of the official base images. See the documentation[5] for more info.
+
+Building the images in this repo can be done with `podman build` as with any
+other application image (note that building with `docker` is not currently
+supported). You need to enable some privileges for technical reasons.
+
+```
+podman build --security-opt=label=disable --cap-add=all --device /dev/fuse \
+  -t localhost/fedora-bootc:40 -f Containerfile.fedora-40
+```
+
+See `Containerfile.fedora-40` for more details.
+
+You are of course also free to fork, customize, and build base images yourself.
+See this page[6] of the documentation for more information.
+
 ## More information
 
 Documentation: <https://docs-bootc-org-fedora-bootc-6570d042ee03afe91a435802a9746be5039.gitlab.io/fedora-bootc/>
@@ -24,3 +43,5 @@ Documentation: <https://docs-bootc-org-fedora-bootc-6570d042ee03afe91a435802a974
 [2]: https://renovatebot.com
 [3]: https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit
 [4]: https://pre-commit.com/
+[5]: https://docs-bootc-org-fedora-bootc-6570d042ee03afe91a435802a9746be5039.gitlab.io/fedora-bootc/building-containers/
+[6]: https://docs-bootc-org-fedora-bootc-6570d042ee03afe91a435802a9746be5039.gitlab.io/fedora-bootc/building-custom-base/