docs: add section for metadata
docs: add section for metadata

jpxiong/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,Lachann/gstreamer,krieger-od/gstreamer,shelsonjava/gstreamer,justinjoy/gstreamer,cfoch/gstreamer,surround-io/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,justinjoy/gstreamer,lovebug356/gstreamer,ensonic/gstreamer,justinjoy/gstreamer,krichter722/gstreamer,lubosz/gstreamer,collects/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,cfoch/gstreamer,collects/gstreamer,centricular/gstreamer,lubosz/gstreamer,justinjoy/gstreamer,lubosz/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,centricular/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,krichter722/gstreamer,cablelabs/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,collects/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,surround-io/gstreamer,lubosz/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,mparis/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer,cablelabs/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,collects/gstreamer,krichter722/gstreamer,mparis/gstreamer,jpxiong/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,krieger-od/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,mparis/gstreamer,lubosz/gstreamer,ensonic/gstreamer,collects/gstreamer,ensonic/gstreamer,krieger-od/gstreamer