[libvirt] [dockerfiles PATCH] README: Provide information about the images

[Andrea Bolognani]

Signed-off-by: Andrea Bolognani <abologna at redhat.com>
---
Note that this document will be displayed both for the
repository containing the Dockerfiles

  https://github.com/libvirt/libvirt-dockerfiles

and for the images produced by said Dockerfiles, eg.

  https://hub.docker.com/r/libvirt/buildenv-centos-7/

so the language is intentionally vague at times. I believe
I've achieved a decent balance between the needs of the two
scenarios, but I'm of course totally open to feedback :)

 README.md | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/README.md b/README.md
index a84caf3..ccd7ad1 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,52 @@
 Docker-based build environments for libvirt
 ===========================================
+
+These images come with all libvirt build dependencies, including
+optional ones, already installed: this makes it possible to run
+something like
+
+    $ docker run \
+      -v $(pwd):/libvirt \
+      -w /libvirt \
+      -it \
+      buildenv-centos-7
+
+from a git clone and start building libvirt right away.
+
+Image availability is influenced by libvirt's
+[platform support policy](https://libvirt.org/platforms.html),
+with the obvious caveat that non-Linux operating systems can't
+be run on top of a Linux kernel and as such are not included.
+
+
+Intended use
+------------
+
+The images are primarily intended for use on
+[Travis CI](https://travis-ci.org/libvirt/libvirt).
+
+The primary CI environment for the libvirt project is hosted on
+[CentOS CI](https://ci.centos.org/view/libvirt/); however, since
+that environment feeds off the `master` branch of the various
+projects, it can only detect issues after the code causing them
+has already been merged.
+
+While testing on Travis CI doesn't cover as many platforms or the
+interactions between as many components, it can be very useful as
+a smoke test of sorts that allows developers to catch mistakes
+before posting patches to the mailing list.
+
+As an alternative, images can be used locally without relying on
+third-party services; in this scenario, the number of platforms
+patches are tested against is only limited by image availability
+and hardware resources.
+
+
+Information about build dependencies
+------------------------------------
+
+The list of build dependencies for libvirt (as well as many
+other virtualization-related projects) is taken from the
+[libvirt-jenkins-ci](https://libvirt.org/git/?p=libvirt-jenkins-ci.git)
+repository, which also contains the tooling used to generate
+Dockerfiles.
-- 
2.17.1