README: update to match current state and add running example

Usage was updated to reflect changes of osbuild arguments. Detailed
information how to run osbuild the first time (hello world example)
was added.

README.md

@@ -68,7 +68,8 @@ The above pipeline has no base and produces a qcow2 image.
 ## Running
 
 ```
-usage: python3 -m osbuild [-h] [--store DIRECTORY] [-l DIRECTORY] -o DIRECTORY
+usage: python3 -m osbuild [-h] [--build-pipeline PIPELINE] [--store DIRECTORY]
+                   [-l DIRECTORY] -o DIRECTORY
                    PIPELINE
 
 Build operating system images
@@ -78,7 +79,10 @@ positional arguments:
 
 optional arguments:
   -h, --help            show this help message and exit
-  --store DIRECTORY   the directory where intermediary os trees are stored
+  --build-pipeline PIPELINE
+                        json file containing the pipeline to create a build
+                        environment
+  --store DIRECTORY     the directory where intermediary os trees are stored
   -l DIRECTORY, --libdir DIRECTORY
                         the directory containing stages, assemblers, and the
                         osbuild library
@@ -88,3 +92,25 @@ required named arguments:
                         provide the empty DIRECTORY as output argument to the
                         last stage
 ```
+
+### Running example
+
+You can build basic qcow2 image of Fedora 30 by running a following command:
+
+```
+sudo python3 -m osbuild -o output --libdir . samples/base-qcow2.json
+```
+
+- Root rights are required because osbuild heavily relies on creating
+  systemd containers and bind mounting.
+
+  It shouldn't interfere with host OS but please be **careful**! It's still under
+  development!
+
+- `--libdir` argument is required because `osbuild` expects itself to be
+  installed in directories under `/usr`. Using this argument you can change
+  the expected path.
+
+- You don't need to use any kind of virtual environment, modern version of
+  Python 3 is enough. `osbuild` uses only standard library and linux commands.
+