bundle.md: clarify arbitrary/conventional dirnames
bundle.md: clarify arbitrary/conventional dirnames

Add a note describing that the names of directories in the bundle are
arbitrary and referenced from the configuration, but conventional names
(like "rootfs" and "signatures") should be used.

wking/opencontainer-specs,HardikDR/specs,LK4D4/specs,wking/opencontainer-runtime-spec,navidshaikh/specs,haijunTan/specs,vbatts/specs,ujuc/specs,opencontainers/runtime-spec,marcosnils/specs,zenlinTechnofreak/specs,runcom/specs,jhjeong-kr/specs,dustymabe/specs,laijs/specs,sladage/specs,philips/specs,cloudfoundry-incubator/specs,rootfs/specs,opencontainers/specs,mheon/specs,gao-feng/specs,wangkirin/specs,ChengTiesheng/specs,talwai/specs,jonboulle/specs,hqhq/runtime-spec,vishh/specs,kensipe/specs,hqhq/specs