Better description of build / install procedure

README.md

@@ -7,11 +7,14 @@ chat: #megapixels:postmarketos.org on matrix
 
 ## Building
 
+Before building this, build and install libmegapixels, libdng and postprocessd.
+
 ```shell-session
 $ meson build
 $ cd build
 $ ninja
 $ sudo ninja install
+$ sudo glib-compile-schemas /usr/local/share/glib-2.0/schemas
 ```
 
 # Post processing