add README

README.md

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+Helper images for building collectd
+===================================
+
+Here is a collection of manifests used to build the Docker images hosted at:
+<https://cloud.docker.com/u/collectd/repository/docker/collectd/ci/tags>
+
+There is one branch per distribution/architecture pair which collectd is tested
+against. Common bits are stored in the master branch, and merged into the
+distro branches when needed.
+
+Each Dockerfile contains a list of package names (mostly support libraries) to
+install, and a list of plugins collectd *master* is expected to build based on
+them. We want to stick to the official repositories, and only install packages
+found there, to ensure collectd releases integrate smoothly in these
+distributions.
+
+Travis-CI takes care of building these images and pushing them to
+cloud.docker.com. NB: Debian/sid and Fedora/rawhide built are triggered once
+per day (as these distributions change constantly). You'll find the build logs
+there: <https://travis-ci.org/collectd/ci-docker/builds>
+
+The goal is to have the resulting images fit in the collectd project's CI, but
+they are free to use for any other case. Only patches related to the CI will be
+considered, though.
+
+Here's an example of how you could build collectd in an Ubuntu/Xenial
+environment:
+```
+git clone https://github.com/collectd/collectd
+docker run -it --rm -v $PWD/collectd:/collectd -w /collectd collectd/ci:xenial_amd64
+./build.sh && ./configure && make && make check
+```