docs: increase recommended disk space for builds (#1450)

With all the packages being built 10GB are not enough anymore.

docs/user/getting_started.rst

@@ -90,7 +90,7 @@ In case of errors read the messages carefully and try to fix the stated issues (
 ``ar71xx-generic`` is the most common target and will generate images for most of the supported hardware.
 To see a complete list of supported targets, call ``make`` without setting ``GLUON_TARGET``.
 
-You should reserve about 10GB of disk space for each `GLUON_TARGET`.
+You should generally reserve 5GB of disk space and additionally about 10GB for each `GLUON_TARGET`.
 
 The built images can be found in the directory `output/images`. Of these, the `factory`
 images are to be used when flashing from the original firmware a device came with,