add guide for adding ci dockerfiles to support more distributions

motivation: easier contribution

changes: add a readme in the swift-ci area to explain how the ci docker setup works and how to contribute

Author: tomerd

README.md

@@ -2,7 +2,7 @@
 
 <img src="https://swift.org/assets/images/swift.svg" alt="Swift logo" height="70" >
 
-### An Ubuntu 16.04 and Ubuntu 18.04 Docker image for [Swift](https://swift.org).
+### Docker images for [Swift](https://swift.org).
 
 #### You can find the Docker Hub repo here: [https://hub.docker.com/_/swift/](https://hub.docker.com/_/swift/)
 

swift-ci/README.md

@@ -0,0 +1,47 @@
+# Continuous Integration Docker images
+
+Swift.org uses Docker based virtual build environment to build & qualify Swift toolchains on Linux.
+
+The Continuous Integration system uses the Dockerfiles in this directory to define the virtual build environment,
+then runs the build and qualification steps inside a docker container based on the image.
+
+## Directory Structure
+
+The Dockerfiles used for Continuous Integration are layed out under the top level `swift-ci` directory.
+Under that, we have a directory for each of the target branches, e.g.
+Continuous Integration for Swift's `master` branch uses the `swift-ci/master` Dockerfiles.
+
+## Continuous Integration
+
+This system is designed to support many distributions.
+Once a working Dockerfile is added to this repository,
+we set up Continuous Integration jobs to produce toolchains for the distribution and publish them on Swift.org.
+
+## Local development & testing
+
+First build & tag the Dockerfile:
+
+```bash
+docker build -f <dockerfile path> . -t <some tag>
+```
+
+Next run the Swift build using that Docker image
+
+```bash
+docker run \
+  --cap-add=SYS_PTRACE \
+  --security-opt seccomp=unconfined \
+  -v <swift source location>:/source \
+  -v <some docker volume name>:/home/build-user \
+  -w /home/build-user/ \
+  <docker image tag from above> \
+  /bin/bash -lc "cp -r /source/* /home/build-user/; ./swift/utils/build-script --preset buildbot_linux"
+```
+
+## Contributions
+
+Contributions via pull requests are welcome and encouraged :)
+
+Focus on mainstream distributions such as Debian, Ubuntu, Fedora, CentOS, RedHat, etc.
+
+Note that the build must run as non-root user.