Adding a simple README

Signed-off-by: Petr Šabata <contyk@redhat.com>

README.md

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+# Atomic Host
+
+Atomic Host is a lightweight, immutable platform, designed with
+the sole purpose of running containerized applications.
+
+## The `atomic` module
+
+The atomic module defines the package set required for
+running a fully featured, module-based Atomic Host.  Unlike
+Host & Platform, the module is not split into userspace and
+hardware-enabling parts.  Instead, it's a well integrated, faster
+moving module capable of booting on various configurations,
+running as an OSTree image and managing containerized
+applications.
+
+`atomic` builds against `bootstrap` & `shim` and requires `shim`
+at runtime.
+
+Only one stream with rolling updates will be supported.  Since no
+other modules build against `atomic`, the module technically
+doesn't need a stable API.  The `platform` module will provide
+the base container image for applications (modules) running on
+top of Atomic Host.
+
+### Initial development
+
+The initial proof-of-concept module was put together using the *Host &
+Platform* tools:
+
+* [Host & Platform content definition](https://github.com/fedora-modularity/hp)
+* [Package lists & module generators](https://github.com/fedora-modularity/baseruntime-package-lists)
+
+The content definition lists packages that define Atomic
+Host, along with SRPM and architecture availability hints.
+This user-curated input is then passed to the package lists &
+module generators that resolve dependencies using a Host &
+Platform development Rawhide snapshot repodata.  The resolved
+per-architecture package lists are then merged, the matching
+dist-git commits found and the modulemd file included in this
+repo is generated.
+
+To fetch the latest modulemd file, simply invoke `make update`
+in this repository.
+
+### Long-term plans
+
+*To be done.*