:/README.md add space reclaim and disk size limit
:/README.md add space reclaim and disk size limit

added description of reclaiming sda3 to section
`disable-auto-update`.

now the limit of `-disk-size-gb` is presented
as a table in the section.

`-oem-size`, `disable-auto-update` and `seal-oem` are
noted to be supported by COS 73 and higher versions.

GoogleCloudPlatform/cos-customizer,GoogleCloudPlatform/cos-customizer