doc: initial documentation

Supply initial documentation for `image-builder`. This documentation
will be imported into the osbuild [1] website.

[1]: https://osbuild.org/

Signed-off-by: Simon de Vlieger <supakeen@redhat.com>

README.md

@@ -37,15 +37,15 @@ We plan to provide rpm packages in fedora as well.
 You can compile the application in `cmd/image-builder` with
 the normal `go` command or use
 
-```
-make build
+```console
+$ make build
 ```
 
 To compile without go build tags you will need to install
 the required RPMs:
 
 ```console
-sudo dnf install gpgme-devel
+$ sudo dnf install gpgme-devel
 ```
 
 ## Prerequisites
@@ -171,10 +171,8 @@ $ image-builder list-images --output=json
     }
   }
 ]
-
 ```
 
-
 ## FAQ
 
 Q: Does this require a backend.