Update README, describing what the orchestrator is

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

README.rst

@@ -1,4 +1,20 @@
 The module build orchestrator for Modularity
 ============================================
 
+The orchestrator coordinates module builds and is responsible for a number of
+tasks:
+
+- Providing an interface for module client-side tooling via which module build
+  submission and build state queries are possible.
+- Verifying the input data (modulemd, RPM SPEC files and others) is available
+  and correct.
+- Preparing the build environment in the supported build systems, such as koji.
+- Scheduling and building of the module components and tracking the build
+  state.
+- Emitting bus messages about all state changes so that other infrastructure
+  services can pick up the work.
+
+Client-side API
+---------------
+
 TBD