README: propose install of libjson-glib-dev
README: propose install of libjson-glib-dev

This avoids
checking for JSON_GLIB... no
configure: error: Package requirements (json-glib-1.0) were not met:
No package 'json-glib-1.0' found

Signed-off-by: Jan Remmet <9c9945a0da87ab3eec6bcf12544e291bfd78ecfe@phytec.de>

rauc/rauc,ejoerns/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc