fedora-helper: Clarify how to generate binary license with dynamic BuildRequires

Signed-off-by: Igor Raits <ignatenkobrain@fedoraproject.org>

tools/fedora-helper.py

@@ -68,6 +68,12 @@ def get_binary_license(ctx, source, add, verbose):
     binary license should be result of combining source license and all used
     crates.
 
+    Combined with dynamic BuildRequires generation, the usage would look like:
+
+        \b
+        rpmbuild -br *.spec -D "_sourcedir $PWD" --without check
+        fedora-helper.py get-binary-license /path/to/buildreqs.nosrc.rpm
+
     This command resolves dependencies and outputs final license(s).
     """
     os.makedirs(CACHEDIR, exist_ok=True)