logd: Add README.property

The properties that logd responds to are:

name                       type default  description

logd.auditd.dmesg           bool  true   selinux audit messages duplicated and

                                         sent on to dmesg log

logd.dgram_qlen.statistics  bool  false  Record dgram_qlen statistics. This

                                         represents a performance impact and

                                         is used to determine the platform's

                                         minimum domain socket network FIFO

                                         size (see source for details) based

                                         on typical load (logcat -S)