Add README file with package documentation

Author: athos-ribeiro

README.md

@@ -0,0 +1,31 @@
+## What is in this repository?
+
+This is an [Open Build Service (OBS) Source
+Service](https://openbuildservice.org/help/manuals/obs-user-guide/cha.obs.source_service.html)
+to allow clang builds for Debian packages.
+
+An OBS instance using this package to rebuild Debian Unstable packages against
+clang is available at [irill8.siege.inria.fr](https://irill8.siege.inria.fr).
+
+## How to use this package?
+
+Since this Source Service is supposed to be used in `buildtime` mode, it must be available
+to the OBS project as a package (just build this package in your OBS project and make sure
+the `Use for Build Flag` is enabled).
+
+Then, you must activate the source service for the target package. To do so,
+read [OBS Source Service
+documentation](https://openbuildservice.org/help/manuals/obs-user-guide/cha.obs.source_service.html).
+In short, you need to add a `_service` file to the target package working
+directory with the following content:
+
+```
+<services>
+  <service name="clang_build" mode="buildtime" />
+</services>
+```
+
+and trigger the target package build.
+
+Check the build log, searching for `Running build time source services...` to
+make sure your source service was triggered.