update README.md and CHANGELOG.md to reflect new bsd boottime collector

Signed-off-by: mischief <mischief@offblast.org>

CHANGELOG.md

@@ -10,6 +10,7 @@ This release contains more breaking changes:
 * [FEATURE]
 * [ENHANCEMENT]
 * [BUGFIX] Count core throttles per core and per package #871
+* [CHANGE] Move `node_boot_time_seconds` from `exec` to new `boottime` collector and enable for Darwin/Dragonfly/FreeBSD/NetBSD/OpenBSD. #901
 
 ## 0.16.0-rc.1 / 2018-04-04
 

README.md

@@ -26,6 +26,7 @@ Name     | Description | OS
 arp | Exposes ARP statistics from `/proc/net/arp`. | Linux
 bcache | Exposes bcache statistics from `/sys/fs/bcache/`. | Linux
 bonding | Exposes the number of configured and active slaves of Linux bonding interfaces. | Linux
+boottime | Exposes system boot time derived from the `kern.boottime` sysctl. | Darwin, Dragonfly, FreeBSD, NetBSD, OpenBSD
 conntrack | Shows conntrack statistics (does nothing if no `/proc/sys/net/netfilter/` present). | Linux
 cpu | Exposes CPU statistics | Darwin, Dragonfly, FreeBSD, Linux
 diskstats | Exposes disk I/O statistics. | Darwin, Linux